Log into the server as ‘root’ and type ‘tail -F /var/log/asterisk/full’
Leave yourself a message.
The stuff that scrolls by on the screen is your call.
Someone asked a very similar question a couple of weeks ago and found out that there was a problem with their mail sending program. The information from your logs should spell out what the error is (as long as it isn’t a problem in the actually sending script, which may fail silently).
If you can’t figure it out (I have confidence in you - you can do this), post that couple of dozen lines around leaving the message and we’ll see what we can see.