Why isn't "my" mail coming?

Question

Mail server

I inherited corporate mail (postfix + qpopper, procmail, mailman, server on SUSE 10 SP2). Domain at the provider, inside ~ 50 users. Two weeks ago, mail began to arrive with "left" addresses (ie, if my address is [email protected]*org*.tomsk.ru, then letters with the address [email protected], etc. may come). And if at first everything fell on the admin, now it has come to the rest of the users.
I've never encountered mail servers, I've been surrounded by books, I'm waiting for enlightenment to come in my brain. But maybe one of the experienced comrades will tell you the solution?

Answer 1

Email has two sender and recipient addresses. Address From: ( or
header.from address) - address from the header of the message, visible to the recipient
, which is reported in the SMTP session between the sender and the recipient
Address To: (or address header.to) - address from the header of the letter, visible to the recipient
Address RCPT TO: (or smtp.to or envelope to) - address of the recipient, which is reported in the SMTP session between sender and recipient
The addresses do not necessarily match. Letters are delivered to envelope addresses. You see the header addresses. When you add, for example, a blind copy - this address is added to the envelope addresses, but not added to the headers.
In this case, the spammers used your address in the SMTP envelope, but a different address in the headers. Why did they do this - apparently there was some reason, for example, the same letter with a valid DKIM signature was used to send to different recipients. Or maybe there was no reason, they just sent it like that.

Answer 2

because it's profitable
, of course, the simplest solution is to switch the domain to some Yandex
, well, don't forget to close the relay, use antispam and follow the logs