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I tried to change my Mail settings away from a external SPAM Filter Service (Upstream) to let my UTM handle all by itself.
What I understand is, that the UTM is opening Port 25 as Proxy and is forwarding the mails to the internal Mailserver and I don't
have to set a Firewallrule for Port 25.
I got curious when a lot of mails didn't get through and got back the SMTP Error 4.4.1 and made a check (https://tools.dnsstuff.com/mstc)
that stated, that Port 25 is down?
Is there anything I am missing in the firewall configuration?
Hallo Bernhard and welcome to the UTM Community!
You shouldn't have to do anything with Firewall rules. Do you continue to have problems after you follow Basic Exchange setup with SMTP Proxy? The WebAdmin portion applies to virtually every mail server.
Cheers - Bob
More is always better. If you have an upstream mail server, keep using it. Then have it forward to UTM for a second look. If a message passes both tests, then forward to your mail server.
The cost of keeping both spam filters in operation is probably money well spent. The downside risk of a successful penetration is not worth trying to save a few pennies on the cost of spam filtering.