1.There are two things in network problem
a. your local area connection in not working.
b. The mail server is not working.
one thing if network has problem then, will you get email bounce back?
If network is down then the page cannot be displayed message displayed.
If your application is sending mail back to you for network problem then for testing this you disable your local area connection and send the mail.
2.for second problem enter any wrong mail id and send the mail. The recipient is not exit the email should be bounce back
If you win you need not explain...But if you lose you should not be there to explain.