I'm a little sad to say our charity is moving over to Office 365, which means that all staff will be moving over to Outlook. We have been using Pegasus Mail since the mid 90's and Mercury for at least 10 years.
This is still in the planning stage, and one of the things I need to do is use IMAP to migrate email accounts from Mercury/32 to Exchange. While we don't have a lot of accounts it would be relatively easy if there was a superuser account - I can't recall ever seeing mention of this, so assume there is not.