Restrictions on sender addresses
smtpd_sender_restrictions

This parameter specifies optional restrictions on sender addresses that SMTP clients can send in MAIL FROM commands.

The default is to permit any sender address. The following restrictions are available:

Restrictions are applied in the order as specified; the first restriction that matches wins.

Specify a list of restrictions, separated by commas and/or whitespace. Continue long lines by starting the next line with whitespace.