problem with gmail server

hi,

i have a problem with the mail app on my macbook air, every time i try sending an email i get a msg that the mail app cannot sent the mail using the gmail server.

i cant sent any mails thru the app and i am sending mails only thru the gmail web account.

receiving mails has no problems, i get all incoming mail to the mail app.

does anybody know how to fix it?

tnx