Skip to content. Skip to navigation

ICTP Portal

You are here: Home FAQ Why did my e-mail bounce back?
Personal tools
Document Actions

Why did my e-mail bounce back?

Determining the reasons for a bounced e-mail

If the ICTP mail server is not able to deliver a message for four hours, it will notify the sender with a message similar to the following:

    **      THIS IS A WARNING MESSAGE ONLY      **

The original message was received at Mon, 25 Oct 1999 23:02:35 +0200
from admin@localhost

The mail software at ICTP was not able to deliver your message.
What follows is the diagnostic information
to try)! 

If you need help, contact the consultants in ICTP Leonardo Building, 
room 2, ext. 353 or 358, e-mail ''. 
Be sure to make available to them the diagnostic information 
included in this message!!

It is important to read further and find out whether there is a transient failure or a permanent error (look out for those "keywords").

Transient failures

   ----- The following addresses had delivery problems -----
abalok@mailhost  (transient failure)
    (expanded from: abalok)

   ----- Transcript of session follows -----
abalok@mailhost... Deferred: Name server: 
  host name lookup failure
Warning: message still undelivered after 4 hours
Will keep trying until message is 5 days old

   ----- Original message follows -----

The relevant information is thus that there is a transient problem, the mail server will keep trying to send the message for five days. Only then, if it doesn't succeed, it will give up and notify you of the delivery failure.

This also means that it doesn't make sense that you send another copy of this message as long as you haven't been notified of the definitive failure.

Problems with addresses: If a message directed to a address bounces back, please send us a notification.

Permanent fatal errors

  • Can't create output
       ----- The following addresses had permanent fatal errors -----
       ----- Transcript of session follows -----
    550 <>... Can't create output
    This happens rarely and indicates that the mail server on the receiving side cannot receive any more messages because its hard disk is filled up. For this reason it is not even possible to notify its system administrators by e-mail. The best thing is to wait a few hours and re-send the message.
  • Message size exceeds fixed maximum message size
       ----- The following addresses had permanent fatal errors -----
       ----- Transcript of session follows -----
    ... while talking to smtp-in.domain:
    >>> MAIL From:<> SIZE=3D4331295
    <<< 552 Message size exceeds fixed maximum message size
    554 <user@host.domain>... Service unavailable
    The recipient's mail server has a limit for the size of incoming messages which you have exceeded, probably due to a big attachment. There are several ways to share files with other people, using AFS, anonymous FTP, or a web page (see section Sharing files). If you really need to send a big file by e-mail, try to split up the attachment in several parts and send them one by one. The following UNIX command divides a file named myfile into files of equal size (1000 KB) with names partaa, partab, partac,...:
       split -b 1000k myfile part
    The recipient can then put the pieces together with
       cat part* >myfile                            in UNIX
       copy partaa + partab + partac + ... myfile   in DOS
    Please note that a copy of every message you send is saved in your sent-mail folder. When you have a huge attachment, it will occupy much of your valuable disk space and therefore you should consider deleting and expunging huge messages from the sent-mail folder.
  • Open spam relay
       ----- The following addresses had permanent fatal errors -----
       ----- Transcript of session follows -----
    ... while talking to
    >>> MAIL From:<user@host.domain>
    <<< 550 Open spam relay - see
    554 <>... Service unavailable""
    The mail server on the receiving side doesn't accept e-mail from your mail server, because it is on a black list. This black list is maintained by and includes all computers which have been abused for unsolicited commercial messages ("e-mail spam"). Please forward the error message to your system administrator who will have to:
    1. Look at where he/she will find instructions to
    2. reconfigure the mail server in a way that protects it from abuse and
    3. have the server be removed from the black list.
  • Too many hops
       ----- The following addresses had permanent fatal errors -----
       ----- Transcript of session follows -----
    554 Too many hops 29 (25 max): from <user@host.domain> via
    [], to <>
    The recipient seems to have installed an infinite forward loop: That is, he/she has set up on host A a forward to host B and vice versa. After travelling back and forth several times the mail server becomes suspicious and returns the mail to the sender.
  • User unknown
       ----- The following addresses had permanent fatal errors -----
       ----- Transcript of session follows -----
    ... while talking to
    >>> RCPT To:<>
    <<< 550 <>... User unknown
    550 <>... User unknown
    In this case there is obviously an error in the address. The host/domain part might be okay, but there is no user abcxyz. It is thus necessary to re-check the address of the recipient. Keep in mind that a very simple error, for example a misplaced or forgotten dot, could be the cause.

Sometimes it's the recipient's fault if a message bounces back to the sender, namely when he or she has installed an erroneous .forward file, which redirects incoming mail to a misspelled or non-existing address. This can be recognized when the e-mail address causing the problem and shown in the error message is not the one originally specified as destination.

If you are not sure what to do, forward us the warning or error message.

There is no need to include the original message that you have written, but you must include the diagnostic information! Without it, we cannot help you in any way.

Powered by Plone This site conforms to the following standards: