СтрадалецъЯ бы и сам не поверил, что MessageID письма будет немного разным, но факт на весь фейс. Хочу попробовать еще считать письма оутлоком и посмотреть на MessageID в этом случае. Пока у меня только совершенно садитский вариант сверки, проверять вышеописанные 4 первых цифры + домен + контрольная сумма вложения.
Message-ID :

Message-ID is a unique identifier for a digital message, most commonly a globally unique identifier used in email and Usenet newsgroups.[1]

Message-IDs are required to have a specific format which is a subset of an email address[2] and to be globally unique. That is, no two different messages must ever have the same Message-ID . A common technique used by many message systems is to use a time and date stamp along with the local host's domain name, e.g., 950124.162336@example.com .[3] On the other hand, if two messages have the same Message-ID, they are presumed to be the same one and one version is discarded. But, as Google MTAs (message transfer agents) mangle the ones of Outlook, several same messages may coexist with original and mangled ones and create problems in referencing each other for threading[citation needed].

Message-IDs, if present, are generated by the client program sending the email (mail user agent, or MUA) or by the first mail server (mail submission agent, or MSA).[4]