Understanding Email Bounce Back Messages With SMTP Codes

Introduction

It is very frustrating to send an email to someone and see the message bounce back. In this tutorial, we will explain the meaning of the most common bounce messages, show where the problem is located, and how to fix it.

For some reason, most users don’t read what is written in the bounce message; they call us and say “the message I sent you bounced back.” This simply doesn’t help, because without reading what is in the bounce message, it is impossible to understand what went wrong and how to fix the problem.

To make things worse, there is no standard for the way each bounce message is written, as each email server’s software uses different wording. In other words, the message that says, “this mailbox doesn’t exist” may be written in dozens of different ways, depending on the software running on the recipient’s email server.

However, each error message is accompanied by a three-digit error code, which is standardized – at least in theory. Unfortunately, some servers use the wrong code to describe the error. For example, the code 550 means “mailbox not available” (i.e., the email address doesn’t exist on that server), but some servers use, incorrectly, the code 554 for that.

Before we move on, one more piece of important information; it is very important to see if you really sent the message that generated the error message. There are viruses and spam software that send emails to random addresses, and sometimes the address they use in the “From” field might be your email address. If that happens, you will get a bounce message if the spam software or virus sent a message to an invalid email address, since your email was the one listed on the “From” field.

So, instead of seeing a legit error message, you might be facing an error message with spam or a virus attached to it. In cases like this, simply delete the error message, as it was not generated by a real email you sent.

There are three basic ways your email address may end up being used to send a virus or spam. The most common is by a random email address generator, meaning that the virus or spam software used your email address in the “From” field of the email, but the email was not sent from your computer or from your web-based email service – you just had the bad luck of having an email address identical to the email address the virus or spam software invented. In this case, there is nothing to worry about.

However, the spam or virus may be sent from your computer, if you have a virus installed on your computer that does that. Therefore, we recommend you run anti-virus program if you see a bounce message in your inbox containing spam or a virus.

The third way a virus or spam may be sent using your email is by someone hacking into your web-based email service, such as yahoo and gmail. If you use a web-based email service and see a bounce message with spam or a virus attached, we recommend you change your password.

Let’s now describe the most common errors, what they mean, where the problem is, and how to fix it.

Invalid Email Account

If you get error 550 (or 554, depending on the recipient’s email server), it means that the email address you are sending the message to does not exist.

Example of messages associated with this error:

550 [email] User unknown

550 Sorry, no mailbox here by that name. (#5.1.1)

550 Requested action not taken: mailbox unavailable

550 5.1.1 [email] is not a valid mailbox

550 No Such User Here

550 Invalid recipient

[email] (user not found)

5.1.0 – Unknown address error 550-‘MAILBOX NOT FOUND’

550-5.1.1 The email account that you tried to reach does not exist.

550 5.1.1 [e-mail]: Recipient address rejected: User unknown in relay recipient table

554 delivery error: This user doesn’t have a [name of the service] account

554 delivery error: Sorry your message to [email] cannot be delivered. This account has been disabled or discontinued

550 #5.1.0 Address rejected [email]

A message that is related to the same problem we are describing is this one, where the recipient’s host (i.e., email server) doesn’t exist:

Sorry, I couldn’t find any host named [host]. (#5.1.2)

If you got the email address from a real person, most likely there is a typo somewhere. Double check the email address you typed. If you obtained the address by phone, you may have misheard the name or a letter and ended up writing down the wrong email address.

However, if you obtained the recipient’s email from a website, from a magazine, an advertisement, etcetera, and you double-checked it to make sure there is no typo, this means that the email simply doesn’t exist. We know how frustrating this may be, but there is nothing we can do about it!

Mailbox Full

If you get any of the messages listed below, this means the recipient’s mailbox is full and cannot receive any additional messages. The three-digit code associated with this error is usually 552. If this is the case, please let the person know by other means (telephone, Facebook or any other form of contact), so he or she can delete old messages and spam and solve the problem. This is a good example of why saying “the message I sent to you bounced back” doesn’t help the recipient, as he or she won’t know why; but saying “hey, your mailbox is full, you must delete some messages” will help him or her to solve the issue.

Mailbox is full

Disk quota exceeded

User is over the quota. You can try again later.

The recipient’s mailbox is full and can’t accept messages now.

Mailbox limit exceeded while appending message

Requested mail action aborted: exceeded storage allocation

Problems with Your Email Server

Let’s now talk about a few error messages you can see if there is something wrong with your email server configuration or if your email server is flagged as a source of spam. If you see any of the messages described below, you must contact the company that is hosting your email server (or the network administrator, in case you are using a corporate email address), so they can solve the issue.

451 Bad reverse DNS.

The “Bad reverse DNS” error message means that the email server you use doesn’t have a reverse DNS configuration, which is mandatory. You must contact the company that is hosting your email server and explain this so they can correctly configure the reverse DNS of the email server. The recipient’s email server is checking this configuration because virtually all servers without a reverse DNS configuration are servers built with the sole purpose of sending out spam.

blocked using b.barracudacentral.org;

http://www.barracudanetworks.com/reputation/?pr=1&ip=%5Bip address]

Mail from [ip address] deferred using Trend Micro Email Reputation database. Please see http://www.mail-abuse.com/cgi-bin/lookup?%5Bip address]

553 User defined policy matched for [ip address]

These messages mean that the IP address of your email server is currently listed as a source of spam on a blacklist. You can follow the link usually provided by the error message to request the removal of the server’s IP address of the list and, after this request is completed by the maintainers of the blacklist, be able to send email to the recipient.

However, there is a good reason your email server’s IP address is blacklisted: someone is using it to send out spam. Therefore, you should report this error to the hosting company of your email server (or to the network administrator, in case you are using a corporate email address), so they can find who is sending out spam using the server, ban the culprit, and then request the proper removal from the blacklist. Otherwise, the IP address will be removed just to be added again, if the source of spam is not contained.

Problems with the Recipient’s Email Server

Now we have some error messages related to bad configuration of the recipient’s email server. If you see a message similar to any of these, you must contact the recipient by other means (telephone, Facebook or any other form of contact) asking the recipient to contact the email server’s hosting company or the network administrator of the company (in the case of corporate emails) to fix the problem. Ideally, you should forward the error message to the recipient using a working email address, so he or she can forward to the person in charge of the email server’s configuration.

CNAME lookup failed temporarily. (#4.4.3)

Sorry, I wasn’t able to establish an SMTP connection. (#4.4.1)

If you get an error message that has something as:

THIS IS A WARNING MESSAGE ONLY YOU DO NOT NEED TO RESEND YOUR MESSAGE

This means that the recipient’s email server is facing a temporary issue, and your email server will keep trying to send the message (usually for the next four days). So, it is safe to simply ignore this error message. After this period (four days), if the message couldn’t be delivered, then your email server will generate a final error message, which you should analyze. Most likely, the problem will be at the recipient’s email server – as long as you didn’t mistype the recipient’s email address and the message ended up at the wrong server.

Of course, there are other error messages that you might see; we focused on the most common ones.

Common Error Messages

Common Error	Possible Cause	Resolution
Mailbox not found Invalid mailbox User unknown Not our customer	Wrong Email address entered	The email address you are trying to send to is no longer needed
Mailbox unavailable Mailbox not found	This may be a problem with the recipient’s email account	Check to make sure you are sending to the correct email address. Wait a while and then try again, if it still bounces you may need to try another method of contact.
Mailbox full Quota exceeded	Your contact has too many emails and needs to make some space before they can recieve yours. This is commonly found on accounts such as Gmail or Hotmail, where account size is limited. Can also be a sign of an abandoned account, where your contact may have stopped checking their account and caused their email account to fill up.	Wait for your contact to empty their email account, or try and contact them through another method.
Host unknown Domain look up failed	This means that the example.com part of the email address does not exist; either through a misspelling or the ISP changing its name. For example, attbi.com changed its name to comcast.com, causing any emails sent to attbi.com addresses to be bounced back.	Check that the email address you are sending to is up to date, and accurate.
Unable to relay	This is a blanket term for a host of different errors that can be possible, such as:
The “From” address might not match an account on the email server	The ISP might require that email comes via a connection (dialup or DSL) actually provided by the ISP – sending using someone else’s connection might not be allowed.	The ISP might require you to authenticate before sending email and you haven’t.
No adequate servers Connection timed out Resources temporarily unavailable Out of memory	These typically related to a problem with a mail server, that you probably don’t have any control over.	They are, in general, temporary, and should resolve themselves over time. Look carefully at the bounce message; the email server involved may continue to automatically try to deliver your email without any action required on your part.
Blacklist Filters	If you see messages that indicate your email was “blocked”, or “listed in”, and references to sites that have things like “spamcop”, “dynablock”, “blackhole”, “spamhaus” and similar in their names, then your email was probably intentionally blocked because the receiving system thinks your ISP’s mail server is a source of spam	Get in touch with your ISP and explain that their server may be on a blacklist somewhere, and then try to use a different email address, or a different email account of your own, to contact your intended recipient. You might also tell your recipient that their ISP is improperly blocking legitimate email
Content Filters	Your email looks too much like a spam message and has been blocked by the content filters.	Scan the bounce for any clues (sometimes there’s more information), and then validate your recipient can get any email by sending a simpler message. Assuming that all works, then re-work your message as best you can to not look like spam.

When a Bounce Isn’t Really a Bounce

Be careful! There’s a class of viruses these days that propagate by “looking like” bounce messages. They instruct you to open an attachment for more information. Don’t. Especially if you don’t recall sending the message in the first place. Don’t open any attachment, especially one accompanying what looks like an email bounce unless you are absolutely positively certain that it’s legitimate.

You may also be getting bounce messages for email you didn’t send. There’s another class of virus that “spoofs” or fakes the “From” address on email messages, and as a result you could be getting bounce messages that have nothing to do with you.

Everything Bouncing?

Finally, if every email you send bounces, then you probably have a different problem. Chances are your email client is mis-configured. Double check your outgoing or “SMTP” server settings, and double check with your ISP to ensure that you have them set correctly.

Various SMTP Error Codes Explained

Error : Wrong or missing login data to access mail transport server smtp.mailhostbox.com. Please check the associated account’s settings/credentials

If your email account is not in ACTIVE status (either SUSPENDED or BLACKLISTED) then you will see the above mentioned error message while sending emails from that email account.

For a suspended email account, you need to contact the administrator of email dashboard. For a blacklisted email account, you need to contact our Abuse helpdesk.

550 5.1.1 Recipient address rejected: User unknown in virtual alias table;

This is the bounce back message that is received by the sender from our server if the destination email address does not exist.

The same error message is received if the recipient has a mail loop. That is, if the recipient has set a forward to another email address and that email address is forwarded back to the original one. In this case, you need to remove the loop in order to receive the emails.

If there is a forward set for an account, and an email is sent to that account, if for some reason the forwarded email is not sent, the original sender may get the above error.
For example, abc@domain.com if forwarded to abc@dom.com, if a third user xyz@dom2.com emails to abc@domain.com, he may get the above error if the forwarded email is bounced.550 5.1.1 Protocol error 
This error code ideally suggests that the sender has used SMTP protocol instead of ESMTP, thus the outbound mail of the sender was rejected. In short, the sending client doesn’t properly communicate and makes errors in the SMTP protocol.
550 5.3.2 Service currently unavailable 
The error code 5.3.2 usually means that the system is not accepting network messages. We would thus need to look outside Exchange for a connectivity problem.
The error code ideally occurs ideally for two reasons which are listed below:
1). The sending IP is blacklisted at an RBL or blacklist monitoring site. (You may verify the same by inputting the IP via this link:http://mxtoolbox.com/blacklists.aspx )
2). There is a temporary block on the IP on our Inbound server due to multiple mails from this IP. Feel free to contact our support desk for more information and regarding de-listing the same.
3). If sending IP does not have a reverse PTR record configured. Reference URL: http://mxtoolbox.com/NetworkTools.aspx
554 5.7.1 Service unavailable; Client host X.X.X.X blocked using zen.spamhaus.org;
This is the error message that is received of the IP address of the sender is listed at zen.spamhaus.org
For further details, please check http://www.spamhaus.org/zen
OR check <a href="http://www.spamhaus.org/query/bl?ip=%3Cyour_ipaddress%3E&quot; style="color: #cc6714; text-decoration: none;" title="http://www.spamhaus.org/query/bl?ip=”>http://www.spamhaus.org/query/bl?ip= and contact zen.spamhaus.org to get the IP De-listed.
550 5.7.1 : Helo command rejected: You aren’t localhost.
550 5.7.1 : Helo command rejected: You aren’t localhost.localdomain.
550 5.7.1 : Helo command rejected: You are not me
Our servers do not accept SMTP HELO command as HELO localhost or HELO localhost.localdomain or HELO . We accept HELO from a valid Domain Name or your computer name which is other than your domain name. Please check with your ISP or Mail administrator for this issue.
554 5.7.1 : Relay access denied;
There are couple of reasons for this error while sending an email

1. If the domain is in the status “Pending Verification” in the control panel.
2. If the user has not checked the option “Our server requires authentication” option while setting up the account. For more information on setting up the email account, please refer: http://support.mailhostbox.com/email-users-guide/configure-your-desktop-…

552 5.3.4 Message size exceeds fixed limit
A 552 email error is typically encountered when there is a problem related to an attachment in your email. Either it has exceeded the size limits of the remote server, or the file-type isn’t allowed by the remote server.
1. For file extensions not allowed on our servers, please do refer to the following link: http://support.mailhostbox.com/node/90
2. For mail (including attachment) sent across from our servers, we allow clients to send upto 30 MB of data.
Solution: Examine the size of the message including attachments. Try zipping the file content.
551 5.7.1 The message was rejected due to classification as Virus,Spam or high bulk ratio. Please see http://support.mailhostbox.com/email-administrators-guide/error-codes
This is a bounce back message that you receive when an email is classified as spam while sending out. If you feel that a genuine email is rejected as spam, you need to send the same email with the full headers and the content to our support team. That email in question will be reviewed manually.
530 5.7.0 Recipient address rejected: Authentication Required.
This error comes when your sending mails to remote domain without using smtp authentication.
550 5.4.5 Recipient address rejected: Hourly domain sending quota exceeded
This is the error message that you get if your domain name exceeds the hourly quota set for the domain name. Here, the term ‘hour’ refers to the last 60 minutes.
550 5.4.6 Recipient address rejected: Hourly sending quota exceeded
This is the error message that you get if the user (that particular email address) exceeds the hourly quota set. Here, the term ‘hour’ refers to the last 60 minutes.
553 5.7.1 Sender address rejected: not owned by user user@domain.com
This is the error message that is received when you are trying to send an email as a different user. This error message is seen by the users who are using Exchange servers with our system.
You need to add an identity for the same in the webmail. For example,

1. Log into the webmail of user1@domain.com
2. Add the identity from the settings tab for user@domain.com (If the from address is user@domain.com). For more information, please refer:http://support.mailhostbox.com/email-users-guide/sender-identities
3. Once the identity is added and confirmed, you should be able to send the emails.

554 5.7.1 : Recipient address rejected: USER IS SUSPENDED 
This error comes if user is suspended in Control Panel.
522 5.7.1 : Recipient address rejected: Requested mail action aborted: exceeded storage allocation
This error comes if the user is overquota.
554 5.7.1 : Sender address rejected: Access denied.
This error comes when the sender address is blocked on our server. Generally this is done if spam has originated from this user.
450 4.1.8 : Sender address rejected: Domain not found
Our inbound servers require that the hostname for the sender server should have valid MX or A records in order to accept emails from it. You need to contact the sending server’s email admin to add valid DNS records for his server.
450 4.7.1 Client host rejected: cannot find your reverse hostname
Our inbound servers require reverse hostname for the sender server to be present in order to accept the email. You need to contact the reverse hostname’s email admin to add the rDNS to into his server.
450 4.7.1 : Recipient address rejected: Policy Rejection- Quota Exceeded
This error comes from our incoming mail server if user is receiving excessive amounts of mails.
450 4.7.1 Recipient address rejected: Access denied. 
This occurs when the recipient address is invalid. i.e., not in form of user@domainname OR if the Recipient address is blocked on server.
451 4.3.5 Server configuration problem – try again later;
451 4.7.1 Service unavailable – try again later;
This error most likely occurs if there is a configration error with our servers. If you encounter any of the above messages, please contact our support helpdesk with the details.
452 4.5.3 Error: too many recipients
The error message is encountered when the user is exceeding the limit of more than 50 recipients in an email. The list of recipients is inclusive of To,Cc and Bcc. If you encounter the above error, reduce the number of recipients in the email and try again.