I hate to be obstinate, but: 1) Most robust "security" of this form is negated if you're running it on a budget VPS. Those things are often extremely and unavo

I hate to be obstinate, but: 1) Most robust "security" of this form is negated if you're running it on a budget VPS. Those things are often extremely and unavoidably insecure for reasons out of your control (out-of-date VM software, insecure control panels, incompetent VM neighbors, etc.). 2) OpenBSD is probably the best option for this. Just use OpenSMTPD and choose a simple secure IMAP server from the ports. OpenBSD is perfectly suited for simple, security-critical applications like mail servers.