[clue] Email server issues

[Yaverot]

My friend has two of his email servers stop working recently. About a week ago.
(by working, I mean receiving email, AFAIK IMAP still works for reading delivered mail)

Port 25 is still open on them according to grc.com's Shields Up.
MX records in DNS are still valid.

Does anyone have a good list of diagnosis steps? Something that might then allow us to google an answer. 

They were working before.
Webpage links gladly accepted as answers.