|You have not provided enough information for us to diagnose your problem.|
If possible, could you tell us if
*What email client are you running
*What settings your client is set to (POP3/IMAP/SMTP)
*Have you configured gmail to allow external clients
*Is this a problem with a new setup, or an existing setup
*Anything else that you think could be related to the problem
I would also suggest logging into your gmail web client and checking that you can send email from there.