I think any questions related to email can be divided into six camps:
Software client questions (i.e. What isn't my email software like Outlook working?) belong on Super User.
Server software installation, email server management, and spam filter installation belongs on Server Fault.
Web-based email issues (i.e. sending email via Gmail) belongs on Web Applications.
Coding based email questions (i.e. Why isn't my mail form sending my email?) belongs at Stack Overflow.
Cpanel email setup, emails sent marked as spam (either through a website or domain controlled by the user), and sending emails from localhost for development belong at Webmasters.
Any questions about how what other sites are doing about email or how their email services work are off-topic.
Serverfault also seems to handle DNS questions related to email but we seem to get a bunch of them here. We should ask their moderators their thoughts on where those may best belong. I'm cool with having them here but they may get faster help over there.