Step 1: Thunderbird is a free and open-source email client application that lets you manage your mail account, news feeds, chat, etc. Most of the users encounter Thunderbird mail error 5.1.1 in Thuderbird while accessing it with Comcast. This error occurs if emails are sent to invalid email addresses.
Step 5:Using this add-on application, you can also remove orphaned news files, clean email accounts, edit email addresses, validate an email address, and so on.