The error is from your providers server. Ask them what is wrong with their email server besides being provided by Yahoo.

Trying to compare web based access to using the POP server only means that you know your username and password.

I do not believe this is frontiers issue. Yesterday I uninstalled one of the frontier accounts unfortunately I had to lose all the saved emails but it was driving me nuts. I reinstalled it, installed the updated special password and it began working. Now this morning both of them will receive but they will not send. I cannot imagine that this is frontiers issue. Every time I try to send an email from either of those two frontier accounts I receive the error message "Login to server smtp.frontier.com failed.". It offers me the option to change the password. Even when I use that option and plug-in the current, correct existing password, it will not send.

Could it be you need to set up the Frontier accounts to use OAuth and generate/set an app-specific password, as mentioned on this page? Chances are they scheduled the change to be effective on May 1st 2017 but it might have happened for you at 07-11.

^{Side note: I have trouble reaching their pages 2 times unless removing/blocking their cookies, probably because I’m not a customer there.}

In general, if an error message reports a server error, it would be safe to assume that’s the case and the issue is temporary. In such cases, do not attempt to change the stored password in Thunderbird right away, as some users may add a second issue in case they forget the original and proper one.

Since posting this question I have done a bit more experimentation on my computers because I have Thunderbird installed on two of the computers I have down here at my store. One in my office and one out front where I work most of the day. The one out front seems to be working fine I can send and receive. I must have done something to the office computer but I just cannot figure it out. I am going to print out a list of the passwords that are in current use in the computer with the Thunderbird program that works and I am going to try to physically copy those into the office computer and see if that fixes it. Just in the two frontier Email programs though. If there is a simpler way that you know I would be grateful to hear about it.

I will post this because it may help somebody else who is as dumb as I am. Apparently what I did in my office computer, and what I am about to explain is actually above my head, is I changed the password to the OAuth password in pop3 and smtp. It appears that the new password needs to appear only in the pop3 location and the smtp location must have the old original password. Now I am not sure why that is but when I changed it back in the office computer everything began working fine.

CornelR said

Now I am not sure why that is but when I changed it back in the office computer everything began working fine.

Because that is how the servers you are connecting to are configured. It is not much different to putting a battery in an LED light back the front. It does not work, unless you do it their way.

Same for email. Mail clients must use the settings created and specified by the server admin. They are in essence the battery in the light scenario. That is why there is always issues with what setting are required, complicated by the fact most email providers in the ISP sphere are very coy about their pop and email settings. So we have folk having to guess based on past experience and "how most do it". In this case frontier have clearly made something of a mess of their update, as you are left guessing what are the correct email setting. One would have though they would have communicated those settings very clearly before the change.

Special Request. Could you please post the troubleshooting information for Thunderbird. (it is on the help menu. Or at least the mail and news accounts part of it. There are questions about how oAuth can work at all with Yahoo as we have been trying for a year to get the appropriate credentials from yahoo so we can implement it.