This was reported in the Wald Message Board ( https://wald.intevation.org/forum/forum.php?thread_id=1815&forum_id=21&group_id=11 )
There it looks like the Save failed after we updated the mail with the encrypted content. Somhow our MIME Attachment is attached but the Body was not wiped.
14:30:03/12660/oomhelp.cpp:invoke_oom_method_with_parms: Method 'Save' invokation failed: 0x80020009
I don't know exactly why this happens for this user or how to prevent it.
To fix this issue I want to implement a check before passing send that the Body is empty through OOM and/or MAPI try to wipe it again and if that fails cancel the send and print an error.