I just published a guide on how to troubleshoot email over at Email Service Guide. It’s fairly straight forward, but does require a bit of technical skills.
The reason why I wrote the guide was actually because I quite frequently found myself Googling for a good IMAP cheat sheet (the RFC is not very good for this). Once I started writing, I realized that it wouldn’t take a whole lot more effort to include POP3 and SMTP as well (since the technique is the same). So I included them too.