These problems will usually be caused by a fault on your device's email client software, a fault on the server or a breakdown in transmission during transit.
When a fault occurs you will usually see an SMTP error message return code.
This article attempts to explain the meaning of the most common error messages return codes.
And how to resolve the resulting problem.
See the links above.
A more complete list of Error Message Return Codes
Here is a link to a more complete list.
The author describes these messages in slightly different terms (but we are talking about the same thing).