I have suspicious message-id header of email sent by php to gmail account:
Message-Id: <5100054f.a489440a.5d93.6a70SMTPIN_ADDED_MISSING@mx.google.com>
Could you please tell does it have this strange format and what SMTPIN_ADDED_MISSING means here? Examples I saw in the internet had format something like this containing sending domain but my message id doesn't contain it for some reason:
38D1C1FD-3C35-4568-925C-FC46CAC0DE8A@sendinghost.com
I don't think I set this header in Zend_Mail. What generates this headers? Do you see any issues with this header?