Ah, ok, so this involves our certificate instead of a postfix mis-configuration of some sort. Yes, our (only) certificate is self-signed, we don't s u b s c r i b e to any SSL Certificate service. On the Mac we can specify "Always trust" when the Mail client first asks about the self-signed certificate; on the PC, Windows 10 Mail also asked about it but didn't offer the same "Always trust" option. Thanks for investigating this and bearing with my lack of technical expertise !
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