Office 365 transportrule that reject mails without encryption.

Office 365 transportrule that reject mails without encryption.
We have a domain secure.xx that only needs to send encrypted mails using a 3rd part supplier using one specific email address.
I would like to create a transportrule that rejects other users from sending mails without encryption.
I know the -ExceptIfMessageTypeMatches "Encrypted" but I can't find a solution to add an "unencrypted" switch.
Any ideas?
macxpresAsked:
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Shreedhar EtteCommented:
I could not understand you request. Please try to re-articulate the question.
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Vasil Michev (MVP)Commented:
What kind of encryption are we talking about here? If you need to ensure (mutual) TLS, best use connectors: https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/use-connectors-to-configure-mail-flow/set-up-connectors-for-secure-mail-flow-with-a-partner

If you want to encrypt the message with RMS/AIP, you can do so with transport rules, and instead of rejecting the message put a "mandatory" encrypt action on it: https://docs.microsoft.com/en-us/office365/securitycompliance/define-mail-flow-rules-to-encrypt-email

For S/MIME or PGP are client-side, you cannot enforce much control there.
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macxpresAuthor Commented:
I'll talk to our supplier of the secure mail system and find a solution. Thanks for replying to my question.
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macxpresAuthor Commented:
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