There is no paid service. Thunderbird is a free product. I really do not know how you got to 50 years of service for Firefox. The product is not yet 20years old and the entity behind the open sourcing of the Netscape browser that developed and sold Netscape was originally launched as a company named Mosaic Communications Corporation on April 4, 1994 SO to get to what I think your issue is. ATT have required, in conjunction with their email provider (yahoo) that the password used so sending and receiving is something called a mail key. See https://www.att.com/support/article/email-support/KM1240308/ My guess is you have used the mailkey for incoming mail but not for outgoing mail. But as the software is installed on your computer, any changes to setting are up to you. No one else has access to your machine.