right+click outbox and pick send unsent messages.

if it doesn't send, there is something wrong with the message - you can delete it or edit it and resend.

Hello Wayne, thanks for your quick reply. I have a SAMSUNG Android & don't see a way to right click on the outbox.

The email app is 9folders. Would it help to uninstall & reinstall the program?

filter feeder

Hello Wayne, thanks for your quick reply. I have a SAMSUNG Android

So this is with a phone?

Yes. I've been running 9folders on my phone for a few years.

I've moved this to android support https://support.mozilla.org/en-US/questions/thunderbird-android

Sorry, I don't have an answer for your problem.

The email app is 9folders.

What do you mean by 9folders? Do you mean the following app? Nine - Email & Calendar https://play.google.com/store/apps/details?id=com.ninefolders.hd3

If the answer is yes, I don't think the above app is related to Thunderbird.