For organisations that have a huge flux of incoming and outgoing mail on their JIRA instances currently, the behaviour in JIRA is such that the mail queue service begins to experience increasing delay (regardless of the actual scheduled delay time) in sending out notification email from JIRA, as your number of mail handlers increase. For big instances of JIRA with hundreds of projects, number of configured Mail Handlers may sometimes be close to a 100 or even more. Such that a mail queue service scheduled to run every minute ends up going every 20mins to an hour, We've been able to find that many times this is often caused by having a large number of mail handlers running as well.

The workaround of increasing the service delay for the Mail Handlers or other services to up to 10mins to improve the frequency of the mail queue may also be inconvenient for some organisations.

Perhaps this can be resolved by putting the Mail Queue service on a different flow from the Mail Handlers.