I can login via IMAP, with SSL, and as soon as I did, I received the automated email message from ISPConfig.
I also tried sending a test email from your server to my personal email and it worked fine. I responded and received the reply at my mailbox @xanderit.com, so all is well.
Cool. So, it was just a matter of switching to Dovecot + Sieve and reinstalling ISPConfig (although, I suspect a reconfiguration would have worked, and maybe that option simply doesn't exist yet).
Surely you will have other questions, but the good news is that they've already been answered one place or another.
Glad we got it all worked-out, and more glad that you learned quite a bit in the process. Cheers!