On 2019-09-02 06:24, Alexander Dalloz via dovecot wrote:

Am 01.09.2019 um 14:41 schrieb Aleksandr Mette via dovecot:

4. Forward e-mail

Don't do that nor let your users auto-forward their mail received on
your MX. Else you will end up faster than you think on blacklists as
very likely your server will forward SPAM and gets classified as a
SPAM source.

You have to let users forward their email because this is functionality they expect. The trick is to spam scan all email first, otherwise as Alexander has said, you end up on RBL's.

Its actually a lot harder than this. Most default installations I've seen don't take into account Return-Path notifications (i.e. passing these notifications upstream to the origin), Troubleshooting last-node delivery issues (user created loops causing mailserver Denial of service if Quota Management wasn't properly configured, greylisting, outbound mail suppression) and Abuse (hacked accounts, interspersed third party server that truncate the return path to obfuscate the full origin).

Mishandling any of these can result in lowered IP reputation which would cause you to wind up on an RBL eventually.