I believe you are talking about 'Xfinity' General info : anyone using Xfinity make sure you have logged on to webmail account and selected the checkbox to allow third party programs. https://www.xfinity.com/support/articles/third-party-email-access '''Server Settings for the comcast mail account:''' Xfinity Email service requires an Xfinity ID and password when sending messages. SMTP server settings: * Description: type comcast email address * Server Name: smtp.comcast.net * Port: 587 * Connection Security: STARTTLS * Authentication Method: Normal Password * User Name = full comcast email address. https://www.xfinity.com/support/articles/email-client-programs-with-xfinity-email Have you manually updated the stored password for smtp ? * Menu icon >Setttings > Privacy & security * 'Passwords' section * click on 'Saved Passwords' * click on 'Show Passwords' You should see two lines for the comcast email address. Either mailbox://account... or imap://account.... and smtp://account..... * Right click on the smtp:// line and select 'Edit Password' * Completely clear all the contents and then carefully type the correct password. It should be identical to the password stored in the mailbox:// or imap:// line. * click on Close Exit Thunderbird Wait a few moments then Start Thunderbird. You need to make sure the account receives an email first, so if you can send an email from another mail account to the comcast email address, then in Thunderbird click on the Get Messages to download it. Or if this is not possible because you only have the one email address - logon to the webmail account and try to send an email to yourself. Once done then try to send an email as normal.