I have a hybrid environment and all my mailboxes are on Office 365. I have a local 2013 Exchange server that has is an SMTP relay server for MFD's, Voicemail, UPS etc. Every so often it just stops working for days on end then starts up again. So far I have restarted several times. There is always a service that I have to start 'Microsoft Front end transport 'after a reboot although it is set to auto.
I can telnet to the server on port 25 and send an email which get's queued for delivery but never gets to it's destination.
What should I be looking for to diagnose this?