Mail, IMCEAEX error

If you receive a non-delivery report about IMCEAEX in reply to your mail, please see

FAQ over contacten
At this moment 20190314 in Dutch only,
but an "ENGLISH" link may be added on that page in the meantime.

which is also about outdated nick name caches.


The article below was written on the occasion of migrating mailboxes from to

By now, all nick name caches that became outdated by this migration
or some course of history before that,
will be cleared.
New IMCEAEX errors may still occur but very probably not for the reasons in the article below.

For the senders

If sending mail from

to a

suddenly does not work but triggers a delivery report including lines such as:

Generating server:

IMCEAEX- O=TUE OU=TUE+20Campus cn=Recipients

#550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##

then follow these instructions or the these in more detail , deleting all of the matches for the concerned recipient.

For the recipient

And if you are the intended

Point the exchange users, all your

to this page. They all potentially have entries in their automatic completion caches that no longer work.

Unfortunately, this is a problem on the mailclients, the computers of the senders, not on the sending mailserver of and not on the receiving mailserver of

So there is no way for system administrators to solve the problem on the servers.

For both:

We found that sending mail from other origins than to the concerned still works, so the problem is on the sending side.

In more detail:


You, as sender of mail to
  • using one of:
    • Outlook
    • Outlook Web Access, visiting using Explorer
  • from inside the TUE campus, the subnet
    (or from a VPN connection to that subnet)
  • in the past
    have entered the using the automatic completion cache, that is:
    • Type a few starting letters for the address
    • One or more matches appear
    • the desired recipient address was clicked
  • now
    • send another mail to the same,
      applying that automatic completion cache again
    • suddenly get a delivery error message including:
      IMCEAEX- O=TUE OU=TUE+20Campus cn=Recipients
      where this account is a valid account name of the intended recipient


Remove the entries for the recipient from your automatic completion cache, as follows:
  1. Type again a few starting letters for the recipient in the To: field, until one or more matches appear.
  2. Select but do not click a match
    for the concerned recipient.
  3. Now delete the selected entry by typing the Delete key.
    On Outlook 2010, you can also click the delete option that should appear on the entries.
  4. Repeat this procedure for all of the matching entries for the concerned recipient.

    This is needed because you cannot see which of them are invalid and will cause the error again.


You should be able to send mail to the concerned recipient by re-entering the address:
  • selecting from the address book
  • typing manually
That address will appear next time in your automatic completion cache and the new entry will work.

Possible causes

The mail delivery problem is a result of some course of history, which we still do not understand. We think that changes in the Exchange environment play a role:

  • The mail contacts for many were removed in July 2016, after we found that they were not needed to relay mail to these recipients and many of the addresses were outdated.
  • In the past, we had custom recipients to relay mail to When they were removed or replaced by an mailbox account, the delivery problem sometimes also occured.
  • A change of Exchange versions

See also

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