@apsturk
I don't use SimpleMail myself because I do my email on a PC (via RDesktop on my X1000, in fact) - I need proper HTML email, (which is something no Amigaoid system offers) and have inboxes of several thousand mails. However, last time I did use it it a few years ago it was quite stable, and nowhere near as bad as you're saying. It may have suddenly got much worse, but it seems unlikely.
I would say, though, that going onto a forum and complaining loudly about how utterly rubbish someone's software is, and how it's a "zero out of ten at the very, very best" isn't going to make him do anything about it.
Work with the developer. Tell him what happens. Tell him the bits you like, and the bits you don't like. If it crashes, tell him what you did to make it crash. Tell him how to reproduce the fault. If you just write an email and say "your program is rubbish and I hate it" he'll just immediately delete the email, whether your grievances are justified or not. Be polite, and help him to help you - then everyone benefits, especially you.