Using my own Postfix, filtering spam and getting all the mail into my ISP's inbox

Question

I currently own a domain bought via GoDaddy.com, which provides me a basic email setup for the most common needs. I configured it to forward all mail to me@mydomain.com to me@myisp.com. I also own a virtual server with a running Postfix that I use for a specific website (all mail to somedomain.com gets forwarded via LMTP to a program written by me).

Since I'm recently experiencing some harassing by spammers, since GoDaddy doesn't seem to filter spam, and since my Windows Phone's Pocket Outlook cannot filter spam, I would like to use SpamAssassin to filter inbound spam by changing my domain's MX records to my server

My ideal setup is the following:

1. All mail delivered to somedomain.com gets redirected via LMTP as usual via virtual transport without any spam check
2. All mail to me@mydomain.com gets redirected to me@myisp.com after a severe spam check
3. I don't care about otheraddress@mydomain.com since I use just one address for now
4. I would like to train SpamAssassin with customized spam rules, possibly based on the presence of certain keywords (links to certain unsubscribe pages I found recurring)

I currently configured Postfix with

transport

somedomain.com                 lmtp:[127.0.0.1]:8025
.somedomain.com                error: Cannot accept mail for this domain

relay

somedomain.com                 OK

(I guess I should add mydomain.com OK too)

virtual

@mydomain.com                  me@myisp.com

(looks like a catch-all rule, it's OK as requirement 3)

I installed SpamAssassin, I can do rcspamd start and set it to boot with the server, but I don't know if there is anything else to do for use in Postfix, and how to apply requirement 1 (only mail to mydomain.com gets filtered)

I also tried to send an email via Telnet to make sure my settings are ready for MX change. I received the message into my account but I found that it gone through secureserver.net, like Postfix didn't rewrite the destination but simply relayed the message.

Thank you in advance. I'm no expert in SpamAssassin, and I have little experience in Postfix (enough to avoid making my server an open relay)

Answer 1

Have you set SpamAssassin as a content-filter or smtpd_proxy_filter?

Postfix needs to know how to talk to SpamAssassin. I've always used Postfix with some kind of SMTP-Proxy (like Amavis or VBoxAdm) so I'm not sure whether SpamAssassin ships with a SMTP-Proxy of its own. Figure this out and use content-filter if you want Post-Queue filtering or smtpd_proxy_filter if you want Pre-Queue filtering. In the case of the pre-queue filter the smtp proxy needs a way to re-inject the mail, so you have to create a new transport in the master.cf. A content-filter is set in the main.cf while a smtpd_proxy_filter must be set in the master.cf.