Active-passive HA cluster does not switch to the backup unit after a failure.
If an individual service has failed that does not disrupt the HA heartbeat, an active-passive HA cluster may not fail over. For example, it is possible that one or more services (such as SMTP, IMAP, POP3, web access, or a hard drive or network interface) could fail on the primary unit without affecting the HA heartbeat.
To cause failover when an individual service fails, configure service monitoring (see Configuring service-based failover) on both the primary unit and backup unit.
Mail queues do not appear on the HA backup unit.
In order to display queue content in the backup unit, mail data must be synchronized from the primary unit. If the Backup MTA queue directories option is disabled, mail queues will not be synchronized. You can enable MTA spool synchronization to view the mail queues from either the backup unit or the primary unit.
Synchronization of MTA spool directories can result in decreased performance, and may not let you to view all email in the mail queues, as mail queue content can change more rapidly than synchronization occurs.