Email Delivery Failed

Deliver Failed:

This page is to help explain how to read a “Delivery failure” message.

How to read a bounce message:

The message is in a standard and slightly obscure format, intended to be read by both humans and robots, the key line to examine is this one:


That will tell you the address the server was trying to deliver the message to, and the reason it failed.

The wording of the ‘reason’ should indicate if the problem was on the mail server reporting the problem or on the mail server it was trying to deliver your message to. Always consider this point carefully so that you try and fix the problem on the right mail server (or by emailing the correct mail server administrator)

Invalid Domain (

This means the name server was unable to find an ‘mx’ record for the domain in question. There are many possible causes for this error. Including:

  1. The domain name may be misspelled
  2. The domain name may have a trailing space or other unintentional characters
  3. The domain may have MX records which point to names that don’t have ‘A’ records.
  4. The domain may not exist!

To check use nslookup, like this:

> nslookup
set type=mx

open (n.n.n.n) 23s failed,…

This error usually means the remote system is not running, or you have a network fault somewhere between your mail server and the destination server. Check if you can connect to the remote server manually, e.g.

telnet n.n.n.n 25

Here is the full text of the error:

For <>, open (NNN.NNN.NNN.NNN) 23s failed A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond. 23 sec

MAILBOX NOT FOUND, User unknown, etc…

This generally means you have miss spelled the destination address or the user has changed their address.

Mailbox full, or Quote Exceeded:

This means the destination user has run out of disk quota and cannot receive new messages until they have deleted some. It may mean that the recipient has stopped reading the account completely.

Host unknown, Domain Lookup Failed:

This could be caused by a DNS failure or the destination domain may not exist. More likely you have miss spelled the domain name.

Unable to Relay

This is usually a configuration error at the remote server, often with low priority mx hosts, resending may work, and it’s worth forwarding the message to the administrator (postmaster) at the destination server so they can investigate.

If you get this error during a local send you may need to enable SMTP authentication in your email client.

Temporary Errors: “no adequate servers”, “Connection Timed Out”, “Resources temporarily unavailable.”, “Out of memory”

These are usually temporary errors, it is worth retrying.

Blacklist Filters: ORBS, RBL, SPAMCOP

This usually means your isp’s mail server has been black listed, the problem will probably be fixed in 24 hours.

If you haven’t found your bounce so far, then use google and search for the error you got, it will very likely give you some useful advice on the possible cause.

TCPWrite failed 0/63998, tot=4096603

Tcpwrite errors like this usually indicate a problem when sending the data/body of the email. If the message continually fails with this error then this could be caused by one of the following:

  1. Send speed set to low
  2. A network fault (does the same size message fail to ‘all’ destinations or only some?)
  3. A fault at the receiving server (e.g.’s server would fail on large messages if not sent sufficiently quickly due to a very low data timeout time and some virus/spam filter inbetween)
  4. A virus/spam filter on your machine might be interfering with the data disable any such devices.

For item ‘3’ the workaround is to use a gateway rule to send emails to that server via another server that is directly on the internet (hosted somewhere) rather than behind a slow dsl modem. Or ask the administrator of the destination server to adjust their timeouts or filter so it doesn’t have this fault.

550 xxxxx

A 550 error code means the delivery failed, the reason follows the number and could be almost anything, common reasons are the user or domain doesn’t exist on the destination server.

550 5.7.1 unable to relay error?

This one means the destination mail server doesn’t think the user/domain in question is a local user, and so it’s telling you it won’t ‘relay’ the message to that destination for you.

Step 1) Check you have turned on ‘smtp authentication’ in your email client

Step 2) Email the administrator of the domain in question for further assistance.

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