feat(spf): conduct spf check for inbound smtp

NOTE: currently a failed SPF check does not block the message.
currently set to add header for testing purposes.

MR !6

test/docker-compose.yml

@@ -83,7 +83,7 @@ services:
     restart: unless-stopped
     cpus: 2
     mem_limit: 512MB
-    hostname: mail.nofusscomputing.com
+    hostname: test.example.org
     volumes:
       - mail_store:/srv/mail:rw
       - mail_ssl:/certs