Is there a way I can block or reject emails in OTRS and also not sending out automatic replies to certain specified addresses? We have some problems with some newsletter-junkmails and some endless loops that are filling up our RAW inbox ...

We're using an external POP3 server and I can't set up anything on that side...

What platform are you running OTRS on? If Linux, set up a MTA on your OTRS server and use fetchmail to retrieve the mail from the remote site and deliver to ids on your local system. You can do anything you want at that point – spam filtering, whitelisting, etc, etc. fetchmail retrieves the mail via POP and redelivers it to a local SMTP on your box so you have total control.

If Windows, I’m not sure there’s much you can do. You can front-end a Windows server with a Linux machine and do the same, but it’s a lot more of a PITA.