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Failed Email

There are many reasons why email messages may not succeed in getting through to the intended recipient. These will generally result in your message being returned to you, probably from an address such as 'Postmaster' and with a subject heading like "Delivery Report (failure)". The message will have a lot (at times, an awful lot) of computer-generated messages, hidden somewhere in which will be the reason for the failure.

It is not possible to cover all reasons why email messages may fail, but the more common ones are described below, together with any remedial action you can take. If you are using Nexus and get an error message when you send a message, take a look at the Interpreting failed-delivery reports page.

1. Wrong Site

The most common reason for an email message failing is that the site part of the address (the bit to the right of the "@" sign) is incorrect. You may simply have mis-typed it, have transcribed it wrongly from wherever you saw it, been given it incorrectly in the first place, or the site itself may no longer be a valid address. For whatever reason, the system will return your mailing with an error message something like:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. 
This is a permanent error. The following address(es) failed:
Unrouteable address

If you can spot immediately where the problem lies (in the example, a clear mis-typing of "physics"), simply resend the message using the correct address. The actual returned message will contain your original text, and most email programs allow you to re-send a message, editing it to remove any of the unwanted error messages and header information. If the failure message indicates that you did actually type what you expected, then you probably have the wrong email address. Go back to your original source.

You can check on whether a particular site name is known on the Internet - use the nslookup program, or similar utility.

2. Wrong Username

If the user part of the email address (the bit before the '@' sign) is not recognised by the remote system, then the error message you get back will depend on the client you are using and whether the domain you are emailing is an Oxford domain or not.

If you use and IMAP client, the error will always look something like this:

This message was created automatically by mail delivery software.
   A message that you sent could not be delivered to one or more of its recipients. 
   This is a permanent error. The following address(es) failed:
   SMTP error from remote mail server after RCPT TO:<>:
       host []:
       550 Invalid recipient <> (#5.1.1)

If you use OWA or Outlook and domain you are emailing is not an Oxford domain, the error will look as above. If it is a domain that does belong to Oxford it will look something like this:

          Delivery has failed to these recipients or distribution lists:

          Examine the text below for diagnostic information returned from the remote server. 
          If you have further problems then please contact
          The following organization rejected your message:
          Sent by Microsoft Exchange Server 2007 
          Diagnostic information for administrators:
          Generating server:

 #550-RPT-UNK: Recipient unknown. 
          Search at 
          550 ##

The same reasons for possible failure apply as for errors with the site part - mis-typing or transcription, incorrect original reference and the username no longer being valid. Again, if you can spot the problem immediately, simply re-send the message with the correct address. If not, then you will need to go back to your original source.

3. Recipient's Disk Quota Exceeded

Mail sent to a Nexus recipient, whose mailbox is full (over quota), will be returned to the sender with an error message similar to:

       Delivery has failed to these recipients or distribution lists:
       The recipient's mailbox is full and can't accept messages now. Microsoft Exchange 
       will not try to redeliver this message for you. Please try resending this message 
       later, or contact the recipient directly.
       Sent by Microsoft Exchange Server 2007 

The error message may look different if the recipient is on a mail system other than Nexus. In either case there is nothing you, as the sender, can do until the person has read their accumulated mail and moved it from their mail box until they are below quota and can receive mail again.

4. Blacklisted email

You may find that other people cannot mail you because Oxford rejects email from systems that are on the postmaster maintained blacklist. This small list contains just the worst offenders. See the Junk mail section for further information.

5. Broken mail servers

The way in which mail servers send mail is defined in a set of open-standards documents. Virus writers and junk mail senders often use their own mail server software in order to deliver their viruses, worms and spam.

A lot of this custom software is broken, in that it doesn't adhere to the standards governing mail transfer. We take advantage of this fact by rejecting messages that come from servers that don't play by the rules. This reduces the amount of spam you receive.

Unfortunately, there is a tiny minority of legitimate mail servers which are broken because they haven't been properly configured by their IT staff. We can't tell the difference between badly set up "good" servers and the "bad" servers and so may end up rejecting a small volume of non-junk messages.

A message which is sent from a broken mail server will be rejected with an error message similar to

550-Invalid HELO (foo) used by 
550-See RFC1123 s5.2.5 and RFC2821 s4.1.1.1
550-for the correct argument to use with the EHLO/HELO command.
550 Further details at

This error message contains references to the technical documents which describe how to properly configure this particular part of a mail server.

If one of your correspondents is receiving this error message when trying to mail you, they should contact the administrator of their mail server and ask them to fix it. They will then be able to send you mail.

6. Other Failures

If you cannot sort out the problem, or are unable to decipher the failure message, contact the IT Services Help Centre service. It will help if you can provide them with a copy of the failed message (forward it by email to, removing the actual text of your message to save space and maintain privacy) and give the original reference for the email address you are trying to contact and what the issue seems to be.


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Written by IT Services. Latest revision 5 September 2014