Honestly I'm not sure if I fully understand their points:
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"Entire message data is not always passed..." - so sometimes it works, while sometimes doesn't? At a glance MailKit seems to have the necessary hooks (MEMessageSecurityHandler) though i have not used it myself so can't say if it works as expected or not.
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When they say they'll release a version with 14.1 (or 14.0.1), do they have confirmation from Apple that the said blockers will be resolved in that release? If they need more time to implement this it's fair, but the wording of the 'key points' sounds like there are blockers out of their control, then how could they commit a release date without Apple's commitment?