Thanks for using Apple Support Communities.
Use this article to determine which part of the email "ecosystem" is the cause of the email issue.
For more information on this, take a look at this article:
OS X Mail: Troubleshooting sending and receiving email messages
Best of luck,
Try deleting the password from Keychain Access (Utilities folder). While there you can also run Keychain First Aid under the Keychain Access menu.
Test. If the problem persists try resetting the keychain.
It may be a server problem.
If you can access your non-functioning account through pacbell's web site, see if you can sign in there. I had an account that refused to recognize a password when Verizon changed their server configuration and I had to get tech support to change it, give me the new password, and then I could sign in to change the password to something I wanted. My other accounts worked okay.