I've been using SimpleMail for years, and never had any closing issues. Maybe we have some different MUI libraries or so in use? Or use it in a different way otherwise? I'm using POP3, not IMAP...

Old versions did trash memory because of AmiSSL, but the recent OpenSSL versions behave much better. SimpleMail does its job for me, but of course would love to get a new option too. I still don't trust it 100% even though can't exactly blame it either.