As reported in the Forum https://wald.intevation.org/forum/message.php?msg_id=5714
and noticed by myself.
Sometimes even after viewing a signed / encrypted mail the messagelist icon in Outlook is not properly updated to the GpgOL icon.
Figure out why that is the case. We change the message class as a first step when we see a mail that is encrypted / signed for GpgOL to handle. So the Icon should also be updated as we have registered it for our message classes.
Maybe an explicit save is missing after changing the message class so that it's only sometimes saved?