From MozillaZine Knowledge Base
(Redirected from Thunderbird : FAQs : SMTP Authentication)
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:
Thunderbird will ask you for your password the first time you send email and you can optionally save it at that time.