I have the same problem.

Let me guess, more Yahoo customers?

Yes - are you able to help?

Find a new povider. Yahoo is a mess.

http://downdetector.com/status/yahoo-mail

Or

http://outage.report/yahoo-mail

I was wondering what makes a contributor 'Top 10'...

What is that supposed to mean? You want me to give you a solution to the horrendous service Yahoo provides?

You regularly tell people to 'ask their provider', as if Thunderbird is never at fault. Yahoo sends an error message and Thunderbird dumps the mail in the 'Sent' folder. Don't Mozilla need to address that?

I've used Yahoo for 20 years, all over the world and for free. They have 225m+ daily users, it's inevitable there will be problems. I've been through 3 smart TVs this year, have a mobile phone that won't run Firefox, a Mac Mini that the fan went on in under 12 months. Tech has problems, but your 'it's not our fault' moan about other providers isn't helpful.

In fact, you may well put off more people from using Thunderbird than you help. Is that what Mozilla want?

Maybe because I know enough about email to decide if it is a provider problem, like this one, and something to do with Thunderbird. There were just as many people with his issue using phones and iPads as were using Thunderbird. In case one of the thing you know it all about is not what Thunderbird runs on, it is not phones or iPads.

Thunderbird is no longer a Mozilla product so I do not think they care. No one will miss you using their free software so do anything you like. Have a great day.

What? https://www.mozilla.org/en-GB/thunderbird/ They may have dropped development, but it's still up there on the Mozilla site.

And, you ignore my point about Thunderbird erroneously putting unsent mail in the sent folder.

And you seem to unaware of how email works. Thunderbird hands the message to your providers server. If it accepts it Thunderbird, or any other email client, consider it sent. What your provider does or does not do with it is out of Thunderbirds control.

I am not going to waste anymore time with you. Maybe your local community college has some basic computer classes you could check into.

It hasn't accepted it, it's sent a message back to Thunderbird to say there is a problem. Thunderbird is incorrectly assuming it's sent before the response from Yahoo servers.

No wonder Mozilla have given up on this software if it relies on the likes of you to manage its support.