Originally Posted by bschorr-MVP

Well, you can try setting it to every minute - but when you do that (espeically if all 6 accounts are on the same server) you're opening 6 connections and if any of the accounts are particularly busy you may end up in a situation where it doesn't finish downloading headers for an account before it begins the next check.

I don't know how Thunderbird handles their IMAP processing. You can try every minute with Outlook and it might work fine. Personally I don't find it that big a deal to wait 5 minutes for e-mail messages espeically if it means less chance of issues with my system.