smtpd_recipient_restrictions= permit_sasl_authenticated, permit_mynetworks ...
smtpd_sender_restrictions = permit_mynetworks, reject_sender_login_mismatch ...

To: support@mydomain.com
From: <support@mydomain.com>
Return-Path: <evil@spammers.ru>

telnet mydomain.com 25
...
Escape character is '^]'.
220 mydomain.com ESMTP Postfix
EHLO spammers.ru
...
MAIL FROM:<evil@spammers.ru>
250 2.1.0 Ok
RCPT TO:<support@mydomain.com>
250 2.1.5 Ok
DATA
354 End data with <CR><LF>.<CR><LF>
To: support@mydomain.com
From: <support@mydomain.com>