Strange Mail message - what to do?

[Ombligo]

I have set up my son with a gmail account and set up his Mail program for it. Starting a few days ago he isn't able to send any messages - he gets a pop up message that I've never seen before and cannot get rid of so he can send mail.

The message is:

Error
This message could not be delivered and will remain
in your Outbox until it can be delivered.

The sender address *******@gmail.com was rejected by the server.



I've checked settings and can find anything wrong. I can also access the account via the gmail website. he is set up as a pop account in gmail and everything ran smooth until a few days ago.

Any suggestions?