I saw some interesting behavior today that resulted in JIRA outgoing email not being sent for 18 hours, so there were 26K mail items to send. The outgoing mail log had an error about the mail queue being considered stuck, which apparently stops all mail from being sent. So I bypassed that first stuck mail item and flushed the queue. The mail queue then grew to 45K mail items before JIRA was able to get ahead of the flood!
There were no large bulk updates or scripts making large numbers of edits or huge subscriptions that I could see. Does anyone have ideas on what might have caused this? I'm scratching my head at the moment.
Also, the mail queue page doesn't show 45K lines of mail items, but I couldn't find where the limit is set on how many it does show. Perhaps something about webwork iterator has a limit?