Why does mail delivery go slow or hang sometimes?
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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
      disappear. 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.
