can receive but not send email

Sounds like the SMTP (outgoing mail) password may be incorrect. To verify go to Mail > Window > Connection Doctor. If that is the problem, go to Mail > Settings > Accounts and select you Mail account. Then choose Server Settings, you can enter the password there. If you don't know the password for your mail server, you will have to login to your mail provider using Safari to reset the password and then enter it in the Server Settings from above.