I've got a couple of symptoms here. Office 365 running the 2013 versions.
1) When I create an email and place it in the outbox it often switches immediately from an envelope symbol to the open letter symbol. Sometimes if I copy/paste the contents to a fresh email I can send it. Opening one of these emails and pressing send results in the same flip/flop again.
2) I figured I had PST problems and tried to run Scanpst. The process runs, stays stuck in ram for a while, and then the process disappears. I never get a dialog box.
Any suggestions as to where to look? Should I update to the Office 2016 that's available, or while I just carry trouble over?
1. Something - usually an AV scanner, some versions of OneNote integration is also known to do it - is 'reading' the outgoing mail when you look in the outbox. Stop looking in the Outbox or stop using that software. If Bitdefender installs an outlook addin, that is the likely cause - stop using the addin, it's not necessary and adds little value anyway. After viewing the Outbox, messages won't send
2. How big is the pst? Any errors in the event viewer?
The PST file is only a couple hundred Mb in size; I move email after I read it to a different PST. I have 8-9 PST files attached in Outlook. Total aggregate size is maybe 2Gb. This has worked fine for years. As for event log, the only thing I see is warnings on 2 PST files about detecting a catalog checkpoint. It could have been doing this for years.
I will search Outlook add-in again. The add-in section in Outlook options lists Evernote clipper, Evernote contact connector, gsyncit, MS Exchange, and TechHit Simplyfile. Others are listed but disabled, including OneNote Notes.