We were having a problem with new mailboxes or new mail address aliases that we've added to our Exchange server in the past few weeks. None of them were receiving external email. We could email them internally just fine. But, external email to those addresses was being rejected with the following bounce message: 554 5.7.1 <email address> Recipient address rejected: Access Denied. I thought this might be a problem with syncing active directory as we had recently done an administrator password change. So, I changed the password in the sync agent and did a sync manually. It completed successfully. Now external email is reaching the intended recipients. So, if you have this problem, and you use active directory sync, make sure the sync is still working.
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