1. Verify that your settings are correct:
a) Server is smtp.gmail.com or smtp.googlemail.com
b) SSL or TSL is enabled
c) Outgoing port is 465, 587, or 25
d) Outgoing server authentication is enabled (login same as incoming)
2. Check if antivirus is interfering. Disable antivirus check of outgoing mail.
3. Run Captcha
http://www.google.com/accounts/DisplayUnlockCaptcha
4. Update your email client to latest configuration. If using Outlook 2002, verify that SP3 is installed.
5. Check if your ISP is blocking Gmail. You may need to use your ISP SMTP server.