There are several reasons for undelivered mails. You can find the most frequent reasons for errors below and some hints on what you can do to solve the problem. We also provide several typical error messages, but
unfortunately, the exact error message differs from mail system to mail system.

  • The mail address is wrong, the part before the at sign “@” does not exist / is unknown. Usually the error message lists the error code 550.Hints: Misspelled e-mail addresses can be found quite frequently in MailWish for the addresses of co-authors, but they are rather rare for the addresses of users, participants or submitting authors.If the wrong address was entered for an co-author, you can simply contact the submitting author and ask him/her to update the e-mail address.

    If you think there is only a small typo in the address, you can also try to search the person and the e-mail address on the web. In a lot of cases, search engines will find the correct address.

    Typical error messages:

    <[email protected]>: host [] said: 550 5.1.1
    <[email protected]>: Recipient address rejected: User unknown in relay
    recipient table (in reply to RCPT TO command)

    <[email protected]>: host[] said: 550 User unknown
    (ID:440:1:2 ( (in reply to RCPT TO command)

    <[email protected]>: host[] said: 550
    Requested action not taken: mailbox unavailable (in reply to RCPT TO command)

    <[email protected]>: host[] said: 550-5.1.1

    email account that you tried to reach does not exist

    . Please try 550-5.1.1
    double-checking the recipient’s email address for typos or 550-5.1.1
    unnecessary spaces. Learn more at 550 5.1.1 u48si4736933wrc.243 – gsmtp (in reply to RCPT
    TO command)

    Your message to [email protected] couldn’t be delivered.
    unknown wasn’t found at

    <[email protected]>: host[] said:
    550 Invalid recipient <[email protected]> (#5.1.1) (in reply to RCPT TO command)

  • The mail box of the recipient is full. This used to be a very common problem, but does not happen very often anymore today, as most providers offer enough storage to their users.Hints: You can try to send the person a very short e-mail to inform about the problem or call him/her etc. The recipient will have to make space in his/her mail account by deleting messages from the mailbox.
    Oftentimes it can be a good idea to follow up some days later again, as another person or colleague with the same problem might have contacted the recipient in the meantime as well.Typical error messages:

    <[email protected]>: host said:
    552 <[email protected]>... Mailbox is full

    <[email protected]>: User is over the quota. You can try again later.

    Mailbox limit exceeded while appending message
    550 <[email protected]>… Can’t create output

    <[email protected]>: host said:
    552 Requested mail action aborted: exceeded storage allocation

  • The mail server does not exist or is (temporarily) unavailable or does not accept e-mails for some other reason.
    In this case, there might be a typo in the part after the at sign “@” or the server is offline or has not been configured correctly.Hints: You can try to contact the person another way, e.g. use another e-mail address or call. If you think that it might have to do with our mail servers, please contact us for help!Typical error messages:

    <[email protected]>: connect to[]:25:
    Connection timed out

    <[email protected]>: Name service error for domain
    Host not found, try again

    <[email protected]>: Host or domain name not found.
    Name service error for name=example.f type=A: Host not found

  • Spam filter problems. Sometimes mail servers reject e-mails from our servers because of some spam server rules.Hints: Usually this is an error in the spam filter, as it does not make much sense to reject all mails from a server or domain name, especially as we don’t send any unsolicited mails and our clients don’t do so,
    either. Furthermore our servers are not listed on any spam server lists.In most cases, the problem has to be fixed by the administrators of the receiving server. Usually it is most efficient if the user contacts the administrators of his/her mail server and asks them to correct the error. Please
    forward the error message as it will help the IT support to understand the problem. In the meantime, we recommend contacting the user you are trying to e-mail by other means until the problem has been solved.

    You can also try to send an e-mail with the delivery error message to “[email protected]” (where is the domain name of that mail server) and ask them to fix the problem.

    There are two exceptions when you yourself or we have to act to solve the problem.

    a)   The following error message means that the mail server rejects the mails because it does not accept the sender address [email protected]:

    b)   Sometimes there are problems with the mail servers of Microsoft (,, etc.).
    Microsoft is using their own block lists (blacklists) for spam servers and they use intransparent rules for these lists and they are known to have many wrong entries (false positives).

    If this happens, please contact us and we’ll write the Microsoft support and ask them to remove our server from their block list (again).

    Example error message:

    <[email protected]>: host[] said: 550
    SC-001 (SNT004-MC10F3) Unfortunately, messages from weren't
    sent. Please contact your Internet service provider since part of their
    network is on our block list. You can also refer your provider to (in reply to MAIL FROM command)


  • The user you are trying to contact is receiving mail at a rate that 450-4.2.1 prevents additional
    [email protected]: host[]
    said: 450-4.2.1 The user you are trying to contact is receiving mail at a
    rate that 450-4.2.1 prevents additional messages from being delivered.
    Please resend your 450-4.2.1 message at a later time. If the user is able
    to receive mail at that 450-4.2.1 time, your message will be delivered. For
    more information, please 450-4.2.1 visit 450 4.2.1 w138si33038971ybg.321 -
    gsmtp (in reply to RCPT TO command)
    The Gmail recipient is accepting mail at a limited rate. All emails above this rate are blocked.

    Wait until the emails are delivered.
    If you’re sending many emails at a time, or emails to large groups, review Gmail bulk senders guidelines.
    If the issue persists and receiver should receive email, contact Gmail support at

  • Error message from an improperly configured mail server.
    If a mail server is misconfigured or the receiver address has a forward address that has been set incorrectly, it is possible that e-mails cannot be
    delivered despite the existence of the mail address. For example, e-mails may be forwarded in an endless loop. In this case, usually no error code is given, but the sender is informed by e-mail with a “Delivery Status
    Notification” which can look like this:”This is an automatically generated Delivery Status Notification.
    Unable to deliver message to the following recipients, because the message was forwarded more than the maximum allowed times. This could indicate a mail loop.” 
  • Warning messages from the sending mail server when an e-mail cannot be delivered within a certain time. If the MailWish mail server cannot deliver a message within 4 hours, it sends a warning message to the
    sending address which looks like this:
    This is the mail system at host
    Your message could not be delivered for more than 4 hour(s).
    It will be retried until it is 5 day(s) old.
    For further assistance, please send mail to postmaster.
    If you do so, please include this problem report. You can
    delete your own text from the attached returned message.
    Attention, this is only a warning, not an error. The server will still try to deliver the e-mail for five days. If this is not successful, the e-mail will be deleted after 5 days and you will receive an error message (see point
    3 above).

    In general, all e-mails are delivered within seconds so that such messages may occur if a mail server is temporarily unavailable.

    Problems caused by Gmail’s spam filter:
    The spam filter of Google Mail may block suspicious e-mails because of the content or links within the e-mail. Such e-mails are not initially delivered to the recipient, but rejected first and then accepted and delivered after a
    few hours. The spam filter of Google Mail uses “machine learning” to detect spam and dangerous e-mails. In many instances, this filter realizes after some time that an e-mail is not spam or dangerous.

    Sample message from Gmail in such cases:

    <[email protected]>: host[]
    said: 421-4.7.0 [      15] Our system has detected that this
    message is 421-4.7.0 suspicious due to the nature of the content and/or the
    links within. 421-4.7.0 To best protect our users from spam, the message
    has been blocked. 421-4.7.0 Please visit 421 4.7.0 for more information.
    f2si109964lfm.40 - gsmtp (in reply to end of DATA command)