I'm having trouble setting up a custom domain email address. It is not about programming. It is about what to expect from an email provider (custom domains). Where should I ask this question? It will get closed if I ask it in Stack Overflow.
I believe your question would be on-topic on Webmasters Stack Exchange.
They generally answer questions about... well... webmastering, which includes hosting, custom domains and email addresses. I would cite their tour here, however it seems to be rather vague...
Either way, you'll probably be fine to ask your question there. Just make sure it's not opinion based or too broad.
I'm a moderator on Webmasters Stack Exchange. I want to clarify that this particular email question would be on-topic because it is about setting up email for your own domain. So would questions about email marketing for your website.
However, see this post on webmasters meta discussing whether email hosting questions are on-topic. John Conde has an excellent answer where he lists the various Stack Exchange sites that might be appropriate for various email questions:
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 mail 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.