Yes, it’s the old “This is a broken PGP/MIME message from MS-Exchange…” problem again.
This time it happens only when using GpgOL to first read the message.
As in, disable the GpgOL add-in in Outlook → messages don’t break even if you first open them in Outlook and then later in Thunderbird (IMAP) or some such.
Obivous downside that you can’t decrypt/verify in Outlook then.
Messages first opened with GpgOL get the blue padlock icon and keep it even after GpgOL is unloaded and Outlook restarted.
Messages first opened with Thunderbird - or web access - can be reread, decrypted and verified with GpgOL without breakage, but don’t get the padlock icon ever.
Yes, Outlook with GpgOL can be open and doesn’t break the message unless the “first read” is done with it.
(Also seems to include race conditions where “already read” status hasn’t gotten to Outlook in time, which is further evidence that this happens in Outlook and not on the server.)
Gpg4win 3.1.3, Office 365 ProPlus latest monthly with the new look.