From MozillaZine Knowledge Base
If your ISP or e-mail service provider requires SMTP authentication for outgoing mail, you will need to use the "Use name and password" option for that SMTP server. Note: SMTP authentication has nothing to do with using a secure connection or any preferences in your POP or IMAP settings.
To set up Thunderbird (or SeaMonkey) to use authentication with your SMTP server:
- Go to "Tools -> Account Settings -> Outgoing Server (SMTP)".
- (If using SeaMonkey: go to "Edit -> Mail & Newsgroup Account Settings -> Outgoing Server (SMTP)".)
- Select the server and press the Edit... button.
- Check the "Use name and password" option.
- Add your username. Your username might be your full e-mail address (e.g., "firstname.lastname@example.org") or only the part before the "@" ("johndoe"), depending on your service provider; check with them which is correct.
- Click "OK".
Thunderbird will ask you for your password the first time you send email and you can optionally save it at that time.
- AuthSMTP offers a global SMTP mail relay service (paid); it can be useful if you use multiple ISP's (and don't want to be changing your settings all the time) or have other problems sending e-mail.