I have an IMAP account, a POP account and a Gmail account (IMAP) with 11,440 messages in my inbox as well as On My Mac archives of mail dating back to 2003. No issues here.I'm a 5 largely because Mail won't run for me - gets stuck trying to upgrade my Library. Not an issue as it's not my primary machine and I'm finding it otherwise stable, but if people are looking at this score to evaluate whether it's fit for day-to-day use, then my answer would have to be no.