This is a support forum for Mozilla Firefox. You will have to contact your e-mail provider for help. https://support.mozilla.org/en-US/kb/problems-with-email-how-to-find-help

Or are you talking about Thunderbird?

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Your question was moved to the Thunderbird forum.

Assuming you are a home user at BT: if you recently changed your password, that might have been due to BT asking to do so using a link they sent (due to suspicious behavior perhaps?), or for any other of course. Changing the password however would mean you are changing it for your incoming email, not outgoing. After all, your incoming mail is protected by a chosen password - outgoing mail should be protected by your personal internet connection and email address only, if I understand well.

Therefor you should already be using the proper (new) password for the incoming email settings, so it’s best not to change this. Instead, enter your full email address in the appropriate SMTP server settings for BT at the bottom of the list on the left in Thunderbird’s account settings, or just when prompted. You might need to use lowercase characters only.

Also see this and this page.

Im contacted Bt and updated my password but I still can not send emails via Thunderbird ( I can however by logging into BT email) so how do I fix sending emails via Thunderbird as this is mt default email ?

Sorry, apparently I provided some erroneous info above - you actually do need to use your full email address for the username, and the password provided (or the new one) for the SMTP server’s password field in order to send, as indicated here and here.

Make sure you use the SMTP settings located as displayed in this support article and described here, which reflects in Thunderbird as SSL/TLS (!), port 465, Normal password, AND mail.btinternet.com in case you previously used the BT Yahoo server. Also make sure to log out of webmail when trying or when changing the password.