I'm kind of surprised you haven't been using IMAP the whole time. I thought the only reason people still used POP3 is because that's all their ISP offered.
While I had my email server up, I used POP3, so when I transferred everything over to Gmail, it was a basic configuration that I had no desire to change.
I used Mercury 32 (4.01b, released Dec 2004) before I shut down the site, but since it was not updated for a long time and I assumed support had dropped for it (no updates until May 2007), I made the switch over to ArGo's Mail Server. However, the IMAP module for that was optional and so I carried on with POP3. After ArGo stopped putting out their regular mail server and was working on transferring to the .NET version (and wanting more money), I decided to fully switch over to Gmail, but POP3 had it's own issues outlined above.
While I took care of my own server, it was not an issue to check mail every minute through my LAN network.

IMAP is definitely the way to go, especially if you read mail on different devices and/or from different locations, etc.
Mercury looks to be in active development again, so if I did decide to bring back up an email server, I would look at returning to Mr. Harris' fine products, but use IMAP this time around.
