Rspamd

Postfix email architecture pipeline showing spam rejected at the SMTP gate and accepted mail crossing the 250 OK line into the queue and mailboxes

Postfix Email Architecture Demystified: Milter, LMTP, LDA

Milter, LMTP, LDA and Virtual look like competing choices. They are not. The real dividing line in Postfix email architecture is the moment your server says 250 OK and accepts responsibility for a message.