I presently have a Postfix installation happily delivering mail (webmail/IMAP) for 100+ users at our company, broadly divided among several domains (sales, operations, management, etc). Recently, someone leaked (don't know who, doesn't matter) the collective address for the salespeople to a spam harvester, who is now spear-phishing with that because it goes to 30+ sales employees. The forward looks like this:
email@example.com ====> firstname.lastname@example.org, email@example.com, firstname.lastname@example.org ...email@example.com
Each of them are now receiving via that forwarding address much spam to contact HR at (malicious link) to update their W-2 address, or collect a secure message, or somesuch.
The forward was set up with the foreknowledge that such a forward could be misused and probably would be at some point, but political constraints forced its creation before I was able to look into how to limit it, and then it worked and wasn't abused for more than three years. There's probably a commentary on the relative unimportance of my company or an underestimation of human nature in there somewhere.
What I want is only to allow mail to that forwarding address (but not ANY forwarding address, as I have several which are info@ types) only from email addresses in specified domains, i.e. @mycompany.com.
My Postfix installation is using MySQL virtual lookups for domains, forwarding, and users, similar to what's described here and other places on the net. It all works flawlessly at this point, except that some forwards are a little too easy to mail to.
Once again, I only want to limit sender domain for selected forwards, not all forwards. We are presently mitigating this by using the bcc: field when mailing to these lists, so the actual list address doesn't leave the building. This does limit discussion among the recipients via reply-all, not sure if that's a feature or bug at this point.