Yeah rDNS is a useful tool for stopping spam and the like… but it also means that every single email sent takes 26-30 seconds to go out. That’s frankly unacceptable when you are trying to operate a commercial service and vastly undermines any possible benefits from it.

The good news is that it is (fairly) easily resolved. All you have to do is put in a command-line flag to tell the mail server not to perform an rDNS lookup. The hard part is figuring out exactly where to stick it.

Up to version 8, I used to refer to this handy guide, which resolved the problem though in ways I couldn’t quite understand! see here at

http://tyrannical.org/page-14

PLESK 8.4 introduces some major under-the-hood improvements, one of which is a subtle reorganisation of where the commands are stored. After some handy support from our ISP CWCS, we found that the command had moved to here:

/etc/inetd.conf

and all you have to do to switch off rDNS is to insert the following flag after the second occurrence of the phrase “tcp-env” in your inetd.conf.

-Rt0

More info and some other useful tips here

Anatomy of -Rt0
What does -Rt0 mean exactly? Well it’s a command that’s issued to the TCP server and it tells it to

R – Do not attempt to obtain $TCPREMOTEINFO from the remote host.
tn – Give up on the $TCPREMOTEINFO connection attempt after n seconds. (default is 26 seconds ho, hum)

The complete howto

1. Login to your PLESK server using SSH
2. type the command vi /etc/inetd.conf
3. type the letter I (to insert)
4. find the two lines that begin with smtp stream and smtps stream
5. look for the second occurrence of tcp-env
6. type -Rt0 into the line (note zero, not ‘o’)
7. hit [esc]
8. type : (shift-colon)
9. type wq! (meaning write the file and quit from vi)
10. hit [return]

now all you have to do is restart inetd, which is done on Ubuntu by entering the command

/etc/init.d/xinetd restart

You should now find that SMTP mail sending is lovely and quick. Phew!

Caveat
Please note that making some changes in the PLESK domain control panel will reset this command line, so you may need to edit it again if you start to notice delayed SMTP sends