Please post your Troubleshooting Information. At the top right of the Thunderbird window, click the menu button ≡, then select Help > Troubleshooting Information. Press the "Copy text to clipboard button" and paste the information into your reply.

I tried to do as requested but the content has more than the maximum number of 10,000 characters allowed!

Since posting my initial query I have discovered the following:

If, under Account Settings, I insert a Reply-to Address under my yahoo.com account the message won't send and I get the error message shown in my first post above. However, if I delete the Reply-to address either in Account Settings OR in the e-mail I am trying to send, the message goes ok. This does not happen with another, non Yahoo, account I have added to Thunderbird.

Any helpful ideas as to why this might be?

You may want to post your problem to the Thunderbird e2ee mailing list at Topicbox. https://thunderbird.topicbox.com/groups/e2ee

Only realizing this now, your problem doesn't have anything to do with end-to-end encryption, so your post is somewhat off topic there. I don't know why I suggested to post there - my apologies for that.

I saw your post to Topicbox. However, since your problem doesn't have anything to do with end-to-end encryption, your post is somewhat off topic there. I don't know why I suggested to post there - my apologies for that. What I actually meant was raising a bug in Bugzilla. Why? There have been some posts here in this forum recently about problems with OAuth re-authentication for Yahoo accounts, thus there could be a general problem with Yahoo. https://bugzilla.mozilla.org/

Again my apologies for sending you the wrong way.

I cant get yahoo mail to work either this month. Last month it was working fine then my hard drive failed and I had to restore it with an image.

Now yahoo mail does not work, its ok through my yahoo account but thunderbird will not talk to it. Here's what I have tried: 1. deleted and set up a new yahoo account in the client. 2. Made sure my email and username are correct. 3. changed authentication to OATH2 for in and out. 4. used imap.. server for incoming and smpt.. for outgoing. 5. Created app password in yahoo and used it when thunderbird asked for it when checking mail.

didn't work. I either get 'edge..to many requests' or 'error 404' or some other random error code I cant remember.

The error changes depending on whether I am sending or trying to read incoming from TB.