Lately I've been having a problem with an user Exchange account, whenever a specific (lets call it @contoso.com) domain sends an email to this user it is rejected by exchange, but for every other user the emails arrive without a problem.
I checked the email log, and found out that the Sender Filter was impeding the emails from reaching this user mailbox. The Exchange server has the Sender Filter activated, but contoso.com is not on the list of not allowed sender domains.
I also checked this single user mailbox properties and it has no speficic mail flow restriction.
Any idea what could be causing this?