Postfilx milter that rejects/fixes manipulated From:-header
If you run web servers that host virtual domains, you often define PHP settings that force a fixed enevelope-from address. Unfortunately many websites provide contact formulas that take the sender e-mail address and copy that with the PHP mail function to the From:-header.
If you run a mail server that checks SPF/DKIM and finally DMARC, you can get into trouble, if the sender address belongs to a company that enforces a strict DMARC policy. If a remote server (and that could also be yours!) receives such a mail, the mail will be rejected, as your web server (or a relay server that handles mail from the web server) fails the SPF, DKIM and DMARC test.
You provde mail services for customers that deliver their mail over submission. If you have infected PCs where bots are going to send mails over users account, they can fake the sender addresses.
This is a little milter that can i.e. run on a web server or a submission server. In the first scenario described, you may like to run the milter with a fix option. In this mode, it restores the From:-header with the envelope-from address and inserts a ne Reply-To:-header with the former From:-header. This will keep formulas working.
For the second scenario described, you may want to run the milter with a hard reject, if the From:-address was faked.
At the current stage of this milter, it reads a simple map file (I am using Postfix) containing all domains that the mail server is the final destionation. Each time a mail is sent, the From:-header is decoded and the e-mail address is split into local part and domain, where only the domain is taken for comparison. In fact, all domains from the map file are compared to the domain, which makes it easier with subdomains. If a domain was found in the mapped file, mail can pass, else it will either be rejected or modified depended of the mode the milters runs with.
As domain maps might be very large, the file is only read at startup and kept in memory. You can send a -HUP signal to the milter to reload its maps and send a -USR1 signal to dump all domains to syslog (or stdout, if running in debug mode).
For both scenarios described above, you can find a samlpe picture of my current mail system, which demonstrates the usecases
I might integrate SQL servers as well for lookups.