I'm getting a high value of spam with the sender faked as my own email address i.e. from firstname.lastname@example.org
Now all emails from my domain example.com are sent using a particular server. How can I setup a procmail filter (or other filter) to delete all emails purporting to originate from example.com which are not sent through this particular SMTP server?
Or perhaps there's a better way to do this with the SMTP server itself (postfix).
EDIT: the spam is coming from multiple SMTP servers. I guess I just want a rule to say if there is an email of the form email@example.com and it does not come from SMTP server mail.example.com, then the email should be blocked.
EDIT: I'll go with the SPF option, though if someone can also show how it can be done with procmail, I would be interested to learn this also.