DNS and backed up queues

Backed up mail server or gateway queues are usually caused by one of two things. First, the file system is working overtime due to absurdly full directories such as spam-pits or archive directories. With so much disk bandwidth and CPU usage just trying to find storage space on the disk, delivery can come to a crawl, and is reflected in backed up queues.

The other common cause of backed up queues is a misbehaving DNS server. Like most problems, log files are tell tale in this situation. Since a comprehensive mail server will also produce detailed logging information, the reason for backed up queues caused by DNS problems becomes apparent in the logs.

This is seen quite often as support calls are generated wondering why this lousy server isn't working when a simple glance at log files would have revealed the problem before making a fool of ones self with a phone call.

A misbehaving DNS server can cause incoming and outgoing queues to back up. Incoming queues get backed up because blacklist lookups or other spam tests time out. DNS servers can become slow, so setting a short timeout is not always good. But when a DNS server misbehaves, and timeout value is set to 15 seconds before it times out, your inbound queue will no doubt become quite full in relatively little time.

Outbound queue can backup because MX records cannot be looked up. Here too, a 15 second timeout on MX lookups leaves queues backed up. Not only that but you should recall that an outbound email requires two lookups - the MX record, and the A record named in the MX record.

Depending on the condition of a misbehaving DNS server, some queries might be satisfied while others are not. Since a DNS server will typically cache results, it might return good results for queries done prior to any trouble. While any queries that must be looked up from down stream DNS servers could fail. It all depends on the root cause of the problem.

BIND is a very complicated machine and many will agree that it's a bit of a hodge podge of functionalities that were never really designed, but rather added as afterthoughts. Some experts agree - it's amazing that DNS works at all given its evolution.