For those of you having email password issues, I'd suggest:
1) Close your mail client (i.e. Apple Mail, Thunderbird, Outlook, etc).
2) Launch "Applications" > "Utilities" > "Keychain Access.app"
3) Select "login" from the sidebar top left area, then select "Passwords" in lower left area
4) Click into the Search box in the upper right and search for your ISPs server names. For example, type "aol" or "google" or "comcast", etc.
5) Look through the search results for password entries related to your SMTP (outgoing) or IMAP/POP (incoming) mail servers. Select them and delete them.
6) Open your email client and try to receive mail, and then to send mail. You should be prompted to enter your mail username and/or password.
Hopefully this might fix issues for those of you using mail clients that use Keychain for credential storage.