Uninstalling and reinstalling the Thunderbird program rarely resolves issues because the problem is rarely to do with the program itself. Most problems relate to the profile settings. At this point I have no idea whether you have created a pop or imap account, but it must be working to some degree as you can see emails arriving. But as the issue is specifc to sending then you may have the wrong outgoing smtp server settings for the comcast account. Suggest you check the SMTP setting assigned to that comcast account. Right click on account name in Folder Pane and select 'Settings' Account Settings opens bottom right it will say the outgoing smtp server which that account is using. Click on 'Edit SMTP server' button. Check you have the following and edit as required. * Description: Enter email address to help with identification * Outgoing Mail Server Name: '''smtp.comcast.net''' * Port Number: '''465''' * Connection Security: '''SSL/TLS''' * Authentication Method: '''Normal Password''' * User name : '''full email address''' click on OK Then exit and restart Thunderbird.