In 'git.torproject.org/checkout/tor/master/doc/spec/dir-spec.txt' ExitNodes are defined as:
"Exit" -- A router is called an 'Exit' iff it allows exits to at least two of the ports 80, 443, and 6667 and allows exits to at least one /8 address space. I would like to setup my ExitNode for ports 443, 465, 563, 993, 995 (https, ssmtp, nntps, imaps, pop3s) only, but this is not possible. What's the reason behind this? Is there any chance to loose this restriction in one of the next releases?