A little while ago I started getting the odd complaint from a couple of clients about their email being really slow to send or not sending at all. The customers always told us that the mail server was broken but because it was a stable installation that wasn’t my first thought. I had noticed it myself as well but hadn’t really had the time to investigate properly.

Discovering the problem with the mail server

Initially I believed it had something to do with the Thunderbird email client. Every time I sent an email I saw one of the cores on my machine max out. So it seemed highly likely to be client-side.  After poking around on my machine for a bit, I thought I should check the server too. I immediately noticed the line:

May 19 09:20:23 srv postfix/smtpd[20934]: warning: x.x.x.x.dnsbl.njabl.org: RBL lookup error: Host or domain name not found. Name service error for name=x.x.x.x.dnsbl.njabl.org type=A: Host not found, try again

What had happened to the RBL?

A very quick look online for that particular RBL (Real Time Blocklist) takes me to www.njabl.org. Their website which has this plastered at the very top:

March 1, 2013: NJABL is in the process of being shut down. The DNSBL zones have been emptied. After “the Internet” has had some time to remove NJABL from server configs, the NS’s will be pointed off into unallocated space (192.0.2.0/24 TEST-NET-1) to hopefully make the shutdown obvious to those who were slower to notice.

Apparently our mail server (or maybe the admins) are one of those “slow to notice”. No idea why they are shutting down. After removing a little bit from the postfix main.cf file I restarting the service. Now everything works smoothly and quickly again!

How did we fix the behaviour?

The line you need to look for is:

smtpd_client_restrictions = reject_rbl_client dnsbl.njabl.org

You need to remove the text “reject_rbl_client dnsbl.njabl.org” if you have other servers listed.  Alternatively, comment the whole thing out or replace it with a different blocklist such as zen.spamhaus.org