I couldn't find Postfix source code repository, I saw only tarballs. Source control (SCM/VCS) is a MUST have. It's unbelievable that people just work with tarballs without proper source control. It doesn't matter which you use, but it must be available. Sending patches over email is what people did last century...
I would suggest using Git, but any other would be fine too like Mercurial, Bazaar or even Subversion. Also I would suggest using GitHub for code hosting, but Bitbucket or Gitorious would also be fine, or you might host it yourself on your own server (such as GitLab). Is there any particular reason why there's no source code repository? Thanks.