Restrictions on sends in HELO commands
smtpd_helo_restrictions

This parameter specifies optional restrictions on what SMTP clients can send in SMTP HELO and EHLO commands.

The default is to permit everything. 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.