Without an NDR (Non Delivery Report) figuring this out will be extremely difficult.
You can however rule out some obvious causes:
- Typo or spelling mistake on the email address.
You would be surprised to hear that this is the no.1 reason for email not getting delivered!
Check your domain for being Blacklisted
- Server error
If you have your own mail server – is it connecting to the Internet and receiving email?
- Account error
Is the email account working?
- Can your server receive emails?
Microsoft created a tool for checking this: Microsoft Remote Connectivity Analyser
You’ll need an NDR to get any further than this…