I loved Pegasus Mail, back in the day when we where using it at a previous employer. It treated email as text files, because, that what they where. You could edit the file on the server and clean emails up. It was nice and simple. Then we changed to GroupWise, which did group things (useful), but email now was E-Mail, and you couldn’t edit things, there was a huge DB back-end, and it was overkill.
So I’m sorry to hear they are closing their doors.