[Exim] random bounces

The mail that is sent through Exim goes through fine most of the time,
however at random times it bounces mail with "unrouteable mail domain"
error:

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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:

sample_user@???:
unrouteable mail domain "aol.com"
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I witnessed messages where the recipient domain is the same as the sender
domain bounce with the same error. For example, one message was from
user1@??? to user2@???. User1@??? got a bounce
notification (unrouteable mail domain "webtv.net")... but if that domain was
really unrouteable, how was Exim able to send an error notification to a
user of the same domain??

Any ideas on what could be the problem? Also, is there a way to tell Exim to
not try to resolve recipient domains? I've heard that Sendmail has a
feature "accept_unresolvable_domains"... is there an equivalent in Exim?

Your help is appreciated.

Melanie