We've had a couple of users set up subscriptions for certain filters that would be returning less than 100 issues.
The subscription was for all logged-in users, and the filter did use the currentUser() function to generate user specific filter results.
The subscription runs once a week, and when it does, it can cause mail backups for over 2 hours.

So it would be great if one of two things were improved to help us with this:

1. Have an automatic equivalent to the "Bypass currently sending mail" if any mail is taking more than a user defined period of time to send, say 5 minutes (or possibly just use the users existing mail queue flush period)
2. Move the mail queue to be a non-blocking model, where one stuck mail doesn't prevent the mail behind it from being sent.