Why does mail seem to come in more slowly when I use DNSBL checks?
On a very busy system I have noticed that the number of available incoming threads seems to decrease when I enable DNSBL checks. This creates the perception of slower incoming mail on my mail system which is usually receiving over 300, 000 messages per day through SMTP. Why does there seem to be a reduction in the number of available threads?
The DNSBL checks to a certain extent rely on the speed of your connection to the internet. You are connecting to a remote server, checking their records for an IP address and getting a response back. While this cannot be instant, it can be extremely fast. Unfortunately, it can also be extremely slow.
When RBL is switched on, GMS checks with your DNSBL server(s) for each message that is sent to the server. Clearly if the remote servers take a significant time responding or the network link is slow, this will affect the speed with which GMS can respond. That is, the reduced delivery speed of GMS is a direct result of switching on the RBL checking.
Due to initial problems with DNSBL services, build 3037 of GMS was specified to check each new connection in turn. The latest release of GMS has had this restriction removed so that now multiple requests are sent to check IP addresses simultaneously.
Keywords:DNSBL MAPS mail-abuse ORDB ORBS spam prevention