SMTP checks need to be run from the actual mail server/cluster that will be sending emails, using the exact same EHLO greeting, etc. Otherwise you may get both

SMTP checks need to be run from the actual mail server/cluster that will be sending emails, using the exact same EHLO greeting, etc. Otherwise you may get both false positives and negatives due to IP blocks and mail header filters on the receiving end. That's without getting into timeouts and remote-side enforced TLS.