Why does sendmail wait to send deferred messages, and how can it deliver the messages faster?

Issue

• In a number of conditions, such as a loss of connectivity to a wide area network, a DNS resolution issue, a downtime of the relay host (usually defined with the SMART_HOST directive in the sendmail configuration files), sendmail will not be able to deliver messages.
• Those messages are then queued as deferred messages.
• Once working production conditions are restored, messages are not delivered immediately.
• The E-mail service can be impacted by the resulting delay.