Did you change any source code files of Zammad or is this a vanialla installation of Zammad?
I’ve never seen a Zammad installation that would send those commands to a mail server.
Edit: And oh yeah, the production log of that moment in Zammad would be amazing.
Well, you might want to restart your pop3 / imap service of your exchange server.
This is a classical sign that it crashed.
I had to learn that some exchange servers have the issue that the pop3/imap service regulary goes into some undefined state. It looks like it’s running, but if you try to talk to it, it just goes wrong.
Telneting to your mail server on that port you’re trying to use with Zammad will most likely also fail or bring no usefull result. Normally restarting those two services should solve the issue.