Things rarely "just break" - usually something has happened or changed to precipitate the problem. I tend to work from external to internal, because 90% of my problems are caused by someone else
Server
Network path to the server
PC - particularly security software
Email client
My instinct would tell me to look at your place of work first - not just the email server but things like the firewall(s) and router(s). If you get a 100% cast iron guarantee that nothing has been changed, AND you are certain that neither yourself, the cat/dog/hamster, or your PC itself has changed anything, then all you can do is to go over system logs and increase logging verbosity where you can - something will show up.