We finally got to the bottom of a problem we were having at work. We had noticed that our smtp delivery speeds had dropped to a crawl. All signs pointed to the RBL configuration which was odd because I had removed all entries from /var/qmail/control/blacklists thinking that it would disable RBL checking. It turns out after going source spelunking that rblsmtpd has a compiled in default of "rbl.maps.vix.com" if no other blacklists are specified. This RBL source is no longer active and just causes long timeouts each time we get an smtp connection.
We fixed this problem by adding a valid RBL back into /var/qmail/control/blacklists and adding RBLSMTPD="" to all lines in /etc/tcp/tcp.smtp that we wanted to disable RBL checks for. Don't forget to run qmailctl cdb after changing /etc/tcp/tcp.smtp; don't forget to run qmailctl restart after changing /var/qmail/control/blacklists.
Consult http://wiki.qmailtoaster.com/index.php/RBLs for more information. I would have included the information in this post on that wiki page but it appears to be broken.
Major props go to Nick Morgan and Greg Jamerson for the hard work they put into tracking this issue down.
Friday, March 26, 2010
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