Email bounces reasons

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Posted on February 15, 2008


User Bounces

Temporary Problems likely to be fixed soon

Mailbox is full

The most common user problem is a full mailbox. Most systems have a limit on how much email is allow to reside on the server for each user, and once that limit is reached, there is no room to accept new incoming mail. The following are all examples of bounce messages caused by a full mailbox:

<>: User is over the quota.  You can try again later.
<>: host said:
   552 <>... Mailbox is full

procmail: Mailbox limit exceeded while appending message
   550 <>... Can't create output

<>: host said:
   552 Requested mail action aborted: exceeded storage allocation

This error will stop as soon as the recipient makes additional room in their mailbox (usually by removing old messages from the server), so you should probably resend your message a little later. However, especially with free accounts, this message could actually mean the user no longer checks the account; a good rule of thumb is if you continue to receive this error for more than two weeks, it is likely that the account is no longer in use.

Message exceeds size limit

<>: host said:
   552 Message size exceeds fixed maximum message size (5000000)

<>: host said:
   552 message size exceeds maximum message size

<>: host said:
   552 Message size exceeds fixed maximum message size: 5242880 bytes

These errors mean that the size of the message, including all headers, text and attachments, exceeds the domain’s maximum per-message size limit – essentially, that your email is too big to be accepted. You should try to reduce the size of the message, or try to split the email into smaller parts and resend it.

Other types of errors that should be quickly fixed can be caused by users misconfiguring their own email accounts. For instance:

Improper forwarding causing mail loop:

554 Too many hops 27 (25 max): from <>
   via, to <>

In general, a “too many hops” error indicates a mail loop: the user has two accounts, forwarding to each other, which creates an endless loop. A message sent a Pobox account is forwarded to another mailbox; if that account is set to forward to Pobox, then a mail loop is created: mail goes from Pobox to the mailbox back to Pobox back to the mailbox, et cetera, until the loop is ascertained and the message returned to its sender.

Permanent Problems

The most common permanent user problem is, of course, User Unknown. The account you are trying to mail no longer exists – it may have been shut down, or you may have misspelled the username or domain (make sure you check bounce messages carefully). The following are all examples of User Unknown messages:

<>: host said:
		       550 <>... User unknown
<>: host said:
		       550 Requested action not taken: mailbox unavailable

<>: host said:
		       550 5.1.1 <> is not a valid mailbox

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

<>: host said:
		       550 Invalid recipient <>

The message that you sent was undeliverable to the following: (user not found)

If you receive a user unknown bounce, and you have checked to make sure the address is correct, the next thing you should do is try to contact the person you were trying to email by other means. Frequently, people are not aware that they are bouncing email until someone else tells them. Also, do not delete the bounce message. It can help the user and their ISP resolve why they are bouncing mail more quickly.

If you do not have other means to contact the person who is bouncing mail, you probably have a dead email address. Try mailing them one more time (preferably at least a day later) in case it was a technical problem, but after that, stop using that email address. If you run a mailing list, and one of your subscribers starts bouncing mail with this error, remove them from the list.



Domain Bounces

Most of the time, when a domain has an email problem, they learn about it very quickly from their users, and in addition, it is relatively rare for an entire domain to simply disappear, so most domain errors should be considered temporary, and will likely be remedied soon — resending mail a little later is usually your best option.

Connection Timed Out / Connection Refused

<user@>: connect to Connection timed out

A “connection refused,” or “connection timed out” message usually results from high volumes of mail being processed at the time your message was received. This could be due to the server receiving more mail then it is used to, a external attack on a domain (such as a mailbomb) or an internal setup problem, causing the domain’s mail servers to refuse connections or cut connections before a message is fully sent. Mail exchangers are set up to only accept as much mail as they can handle, so when problem is resolved, you will be able to send your mail without problem.

Domain Not Found

<>: Name service error for domain
                     Host not found, try again

A “domain not found” error means that the domain name to which you sent the message does not exist. Usually, this means you misspelled the domain name, but it may indicate a problem with the domain’s record that prevents the domain from being found. There is another Domain Not Found error: “Sender Address Rejected: Domain Not Found”; for more information, please see Domain Not Found: Anti-Spam bounce below.

Relay Access Denied

Another domain error that can be due to either the sender’s or recipient’s domain is a relay access denied error:

<> host said:
    554 <>: Recipient Address rejected:
    Relay access denied

This error indicates that, somehow, a message intended for a address found its way to our mail server, and since our server doesn’t accept mail for, the message was rejected. It is quite rare, actually, to reach a completely incorrect mail server when sending email to a domain, so if you receive this message, the problem is usually a misconfiguration error with the receiving domain (for example, if you receive notice that mail sent to a Pobox user was rejected by a Pobox mail server with this “Recipient Address rejected,” error, this could indicate a problem with our mail servers, since Pobox’s mail servers obviously should accept email for Pobox users. Or, it could indicate a problem where the sender’s system isn’t looking the mail server up correctly).

Another reason for receiving this error is the possibility that a domain has recently changed hosts, and while the change has taken place, the new domain record has not yet propagated fully, and your message reached the old hosting company which no longer accepts mail for the leaving domain. If you receive this error, try resending your message twenty-four hours later.

Another Relay Access Denied error exists: “Sender Address rejected: Relay Access Denied”; this is actually an anti-spam bounce, so please see Relay Access Denied: Anti-Spam bounce below.



Anti-Spam Bounces

System administrators frequently set up their systems to refuse mail from spammers, but since no spam filtering system is perfect, your message may have been caught in the spam block. Some of the most common: Spam blocks

<>: host refused to talk to me:
550 Permission denied

or contains a creative note, such as

<>: connect to
550 Connection refused - we hate spammers!

These errors mean that your provider’s domain name (or specifically, your email address) is explicitly listed as a known spammer on a blacklist. This may be based on an external service that provides blacklists of known spammers to ISPs, or the administrator may have placed a block based on a large volume of mail coming from your domain. Most of the time, your email provider will need to contact the system administrator to have the block removed, so you should contact your provider immediately. Assuming that your provider is, in fact, not a spamming service, they will want to act quickly to get themselves removed from these blacklists.

There are two other anti-spam bounces that are very, very similar to some of the domain errors shown above. The first is a Domain Not Found error, where the sender’s domain could not be found:

<>:host said: 554
   <>: Sender Address Rejected: domain not found

Unlike the Domain Not Found error shown above as a domain error, this is an anti-spam bounce. Notice the “Sender Address Rejected”; this means that the problem is actually with the Sender’s email address – specifically, that the domain used in the sender’s email address was not a valid domain. When a mail server receives an email for one of its users, the server checks to ensure that the sender’s domain is a real domain – if the domain name does not resolve, the message is rejected with the “Sender Address Rejected” error. This is an anti-spam error in that it prevents mail servers from accepting spam where the domain is completely fake, meaning the message could not really have originated there.

The other type of “554 Relay Access Denied” error is a problem with the sender using the wrong mail server to send their message:

<> host said:
    554 <>: Sender Address rejected:
    Relay access denied

Essentially, this error message is similar to a Recipient Address rejected, in that the server reporting the error does not like the domain name; however, in this case, the problem is that the outgoing email server did not like the domain of the sender’s email address (as opposed to the above Recipient errors, where the incoming mail server does not like the recipient’s domain).

What this error generally indicates is that the outgoing server you’re using (usually the one assigned to you by your ISP) will not accept email with the From: address you’ve used on the email. For example, domains such as,, and do not allow users to send email their outgoing mail server if the “From” email address used on the message is not the one assigned by them (e.g. you can’t send out mail that uses your Pobox address as the From: header via their mail servers).

Many ISPs have begun to enforce these restrictions to prevent spammers from using the ISP’s access and SMTP server to send spam. To remedy this error, you will need to use the email address assigned to you by the provider as the From: address on outgoing email.



retry time not reached for any host after a long failure period

The bounce back you have sent to us is not the original bounce back. The error below means the email was resent to the recipient after a failed attempt already occurred. There is a retry timeout in place, which means you can not resend the email within this timespan.