Outlook somehow changes their content type to multipart mixed when they are moved:

10:28:32/1148/mapihelp.cpp:change_message_class_ipm_note: content type is 'multipart/mixed'
10:28:32/1148/mapihelp.cpp:change_message_class_ipm_note: protocol is 'application/pgp-encrypted'

Originally reported here: https://wald.intevation.org/forum/message.php?msg_id=6113