From MozillaZine Knowledge Base
- This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).
If the password is rejected when Thunderbird tries to login to the mail server its usually due to using the wrong username, not the password. It might also be due some other setting in Tools -> Account Settings -> Server Settings, a problem with your anti-virus program, a problem due to upgrading, or having non-alphanumeric characters in the password (the CompuServe POP server rejects passwords that are accepted by webmail). Its probably not due to your firewall - that would either cause some type of connection error or silently fail.
Sending of password did not succeed. Mail server pop.sbcglobal.yahoo.com responded: invalid user/password
- Some email providers expect you to enter the full email address as the username. Others expect just the portion to the left of the '@' in the email address. Don't assume that whatever username you might have used with webmail you can use with an email client.
- Usually if you use the wrong setting for secure connection or secure authentication you will get a error message about it couldn't make the connection or the connection attempt was rejected. However, sometimes you get a misleading error message about an invalid username/password. It might also fail if you're an older mail server that your email provider is phasing out. Check the support page on your email providers web site to see if you're using one of the mail servers it lists.
- Hotmail uses your Windows Live Id to log in. The webmail user interface will let you set a password longer than 16 characters. If you try to login using webmail it will succeed as long as the first 16 characters are correct. It ignores the rest. However, if you use a email client such as Windows Live Mail or Thunderbird with a password longer than 16 characters, it will fail. The workaround is to change your password in webmail to 16 characters or less, delete the saved password in Thunderbird using Tools -> Options -> Security -> Passwords, and restart Thunderbird. Hotmail is the only known email provider with this quirk, but there might be others.  
- If you copied and pasted either the username or password its possible it has a trailing blank space. Thunderbird should ignore that blank space, but it doesn't. One way to detect if there is a trailing blank space is to click on the username or password and then press Control-A to select (and highlight) the entire entry. 
Sending of password did not succeed. Mail server mail.stillbrook.net responded: Your account has been suspended
- Your email provider suspended your account. You need to talk to them to find out why.
Password was not accepted by Earthlink
- You are probably using a copy of an old password. Delete the saved password using Tools -> Options -> Privacy -> Passwords -> Edit Saved Passwords. When you are prompted to enter the password check the check box to save the new password before pressing the OK button.
Note: Changing the password in Thunderbird doesn't actually change the password for your account. It just changes the saved copy that Thunderbird uses so that you don't have to type it in each time. If you really have to change your accounts password you need to use a browser and login to a webmail page on your email providers web site, and then find the appropriate menu command to change the accounts password.
Sending the password did not succeed. Mailserver pop3.blueyonder.co.uk responded : Logon failure: unknown username or bad password
- Some email providers support aliases. You can't use them when logging in, you need to use the real username for the account.
Sending of Password did not succeed. Mail server pop.servername.com responded: access denied
- This is probably a problem with the mail server but it could be due to a copy of a old password. Sometimes its helpful to try to duplicate the problem with another email client such as Outlook Express. If you configure it to leave a copy of the message on the server it won't effect Thunderbird. See this article for how to do that using Outlook Express and this one for how to do that using Windows Mail.
Sometimes you can't solve a problem without seeing what is actually sent between Thunderbird and the mail server. See session logging for various ways to enable logging. Usually you want to use the built-in POP3 or SMTP logging support in Thunderbird, but sometimes its easier to use a packet sniffer or to connect through a proxy server that logs the network traffic to a log file.
You can verify the saved password is correct by looking at it in Tools -> Options -> Security -> Passwords -> Saved Passwords -> Show Passwords.