Apple Mac OS X Server (Administrator’s Guide) - Manual (Page 400)

400 Chapter 9
Note: Reverse-lookups of IP addresses may slow the performance of your mail service
because lookups involve more contact with DNS service.
To check SMTP server names and IP addresses:
1 In Server Settings, click the Internet tab.
2 Click Mail Service and choose Configure Mail Service.
3 Click the Filter tab.
4 Select “Log connection if SMTP name does not match IP address” and then optionally select
“Reject if name does not match address.”
5 Click Save.
Your SMTP mail service may be unable to do a successful reverse lookup of a server that
identifies itself in a nonstandard way. Specifically, the SMTP service can determine the server
name in a HELO command that doesn’t deviate too much from standard form.
The SMTP service can determine the server name and do a reverse lookup from HELO
commands like the following:
helo mail.example.com
helo I am mail.example.com
The SMTP service cannot do a reverse lookup from HELO commands like the following:
helo I’m mail.example.com
helo I am mail server mail.example.com
helo what a wonderful day it is
The following table explains the results for various configurations of the settings for logging
and rejecting unsuccessful reverse lookups.
Log Reject Result
No No Accepts all HELO commands
Yes No Accepts all HELO commands and logs each server whose name doesn’t match
or whose name can’t be determined from the HELO command
Yes Yes Logs and rejects each server whose name doesn’t match or whose name can’t
be determined from the HELO command