E-mail deferral occurs when a message (usually outgoing) cannot be immediately handed off to the next relay host. It is quite common to have messages queued as "deferred" as remote mail servers may not be available for any number of reasons: network traffic congestion, service outages, server load, DNS hiccups, grey-listing, etc.

Sometimes spam and delivery notification messages will get stuck in the queue as well. Spammers send a lot of e-mail, but rarely accept return e-mail, including bounce messages. These messages can get stuck in the queue. PerfectMail™ has automated processes that clean out such messages on an hourly basis.