double-send of mail through microsoft server

This has been going on for a while but I got to filing a ticket only now. One of my mail accounts is on office365.com and I access it from MM through IMAP and SMTP. It works fine. However, when sending, I get 2 entries for each sent email. See the attachment (screenshot of the Sent mailbox with double entries highlighted). They are seemingly the same except for the filesize. Indeed, while the actual mail content of both entries is the same, the headers are different, one file has a bunch of headers more than the other. See attachment (text comparison of raw files in BBEdit). As far as I know, only a single mail is actually sent.