The setup in our group is the following:

Mail server Workgroupmail Enterprice, which is pulling mail from 4 external servers (POP3). Group members get their messages from the mailserver. Server has internal IP 192.168.2.1

Receiving email works fine, but we can not send anymore. Error message says:

"An unknown error was returned from the SMTP server. Subject "Test message", account 'johndoe', Server '192.168.2.1', Protocol: SMTP. Server response: '557 Your IP 192.168.2.1 does not have a reverse DNS entry'. Disconnecting"

Does anyone have any idea what the cause may be?

In the settings, I left the SMTP Listener Socket blank. Any chance that this the cause is?

Maybe I am misunderstanding the operating principle, but Spamfilter is sitting between internet and mail server. How does Spamfilter know on which external servers to check for mail? Am I not supposed to give their address someplace?

Or, is Spamfilter not the right application for our setup?

All help is highly appreciated.

It sounds like you are trying to use Spam Filter as an outgoing email server as well as incoming. This isn't how it is supposed to work, but if you add your internat email server(s) IP's the IP whitelist on SpamFilter it should....

Just a hint, you may want to turn off the 'reject if no RDNS' feature until you get a good handle on SpamFilter because you are going to get a TON of email (legit and spam) that will be blocked becuase of this and you probably don't want to spend all day dealing with it.