mail: cannot send message: process exited with a non-zero status

I've had a cronjob working for about a fortnight without any problems. Then last night I checked I didn't get the email that I usually get. I went to the terminal to try send myself an email, I got the following error: *mail: cannot send message: process exited with a non-zero status* I haven't changed anything with my ssmtp cfg file. It just stopped working, when I check and recheck everything, the code, ssmtp, everything is perfect. I send out my emails twice a day via cronjob. The crontab hasn't been interfered either. I really don't know why it would stop working. The system sends out emails via gmail - I've gone into the gmail account and sent out test emails, they are sent and received without any problems. Additionally I've checked throughout google, forums, websites I don't see any mistakes. This makes sense as everything was working fine 24 hours ago, and now it's just stopped. **Q:** Is there any way of diagnosing and troubleshooting how to solve such a problem?