AT&T now requires secure mail keys in place of account passwords. Create a key on the AT&T website, delete the old passwords from Saved Passwords in TB, restart TB, enter the secure key when prompted.

https://support.mozilla.org/en-US/questions/1288812

Created master PW at Att. Del PW at TB. Created new master PW at TB. Pop up says: Login to server imap.mail.att.net with username jasonwg1214@att.net failed. Retry/enter new PW/ cancel are the choices in pop up box. Any suggestions ? I have win 7 pro. Google Chrome. 2 others in the family have Att.net at TB, with no problems since 2016. safe mode & no add-ons, same prob.

When you create a secure mail key, use it in place of the login password, not the master password, which is only to protect access to the stored passwords. Other accounts that haven't had password changes still work, but if the password is changed, the new passwords won't work - they must be replaced by secure mail keys.

Ty for taking time to help me, but I guess there must be some other prob. as i have a secure mail key and try to log into TB, using same and keep getting the same pop up about: "Looks like this account is already linked to your AT&T Security Key Access ID." I guess I will have to use the web mail prog thru Yahoo for my acct. *** @att.net . I can get to my account, but I can't get to inbox with this pop up dialog box: " Login to server imap.mail.att.net with username *** @att.net failed."

Instead of entering your normal password, you follow the instructions at this link to create a 'secure mail key' password. https://www.att.com/support/article/email-support/KM1240308

Delete the stored passwords in Thunderbird. Then when prompted in Thunderbird, you use that 'secure mail key' password instead of the normal password and remember to select the checkbox for Password Manager to remember password.

TY for taking time to reply to my prob. in TB. The link you sent takes me to this page & info. I dont see any info. about how to secure this Secure Mail Key after arriving at the directed link and selecting " Profile " & then signing in with my ID of *** @att.net and PW. I have no idea what to do as I see nothing related to the phrase 'Secure Mail Key." Go to Profile > Sign-in info. Select the email account that you want to get a secure mail key for. (You’ll find a drop-down menu at the top if you have multiple accounts.) Scroll to Secure mail key and select Manage secure mail key. If you have more than one email address, select the one you want to use. Select Add secure mail key. Enter a nickname for the secure mail key to make it easier to recognize. Select Create secure mail key.

Whew.....what a complicated procedure...it did work. and now i have this secure mail key thingy which is 16 characters long......TY.

Now I have a prob again after doing above and TB worked fine for awhile. When sending mail I get this dialog box pop. " Login to server imap.mail.att.net with username XXXX@att.net failed. /retry/enter new password/cancel are the choices to click on.