Thunderbird will not get mail from some of my addresses

This is on my PC, not a phone. I have 5 different email addresses, all with att.net. Thunderbird still checks 3 of them. On the other two, it says the email server responded with sever error. It has been like this since November. I have checked the password recorded in Thunderbird and it is correct. I have to go online and log in for each email account to check my mail (at currently.com) This happened once before on a different email address. Someone gave me step by step instructions and we created a "key" which Thunderbird has used successfully since. But I did not write down the steps to create a key and have been unable to figure out how to do it. I wish I could understand why this happens. I have tried reentering the password on my Thunderbird app with no change in the problem.