mail.app – Incoming emails going to trash (no rules set)

Lots of incoming mails skip the inbox and go directly to the trash folder (not the junk folder). When I visit my Trash folder I see a huge number of unread messages from legit senders.

  • I have Mail.app linked to my Exchange account for work (this is the only linked account).
  • Messages I send myself via my work’s SMTP server always go to trash.
  • Incoming messages that I manually flag as Junk sometimes go to the Junk folder, and sometimes they show up in the Inbox, but they never go to the Trash folder.
  • I have no rules of any kind set up in Mail.app, and I never did.

My question is, who is the culprit – Mail.app, Exchange, or my work’s SMTP server?
How do I find out?