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# Why does mail delivery go slow or hang sometimes?

There are a number of things that can affect mail delivery. Here are some common causes.

> 1. Murder Master is overloaded. In a normal setup the murder master is queried for each incoming messages
> 2. Delivery.db corruption. A backend will/may reject mail if the delivery.db is corrupt
> 3. Slow dns. Some MTAs (like sendmail) talk to dns a lot. If dns is slow, mail delivery appears slow. We run named on each Cyrus machine
> 4. Lmtpproxyd is having trouble. Sometimes lmtpproxyd will die and dissapear. Sometimes it just hangs. Not sure why, but restarting may help.

If you have a large volume of mail then Cyrus Murder Mail Delivery has some tips.

We experienced some filesystem locking problems when allowing to many lmtpd-processes to be started. Performance was seriously degraded, resulting in timeouts and endless retries. We use a postfix instance between to limit parallel deliveries to lmtpd resulting in much higher throughput. With berkeley-nosync as duplicate suppression db, 50-100 Messages/sec are doable.