I think you will find it is the _ that is the issue really.

I don't understand. The "_" exists in both folder names, but one works and the other doesn't.

Are you saying some combinations of "_" and other characters are problematic? And that no combination of characters, as long as "_" is not one of them, always works?

Folder names with NON ASCII or Filesystem characters like /\|* often are an issue. The underscore if used in Programming to indicate an internal variable name, and therefore your folder name may be getting confused with code. The underscore also has historically been used in text based email ( and some wiki syntax) to format a word as underlined _Underlined_ word so you could run into parsing issues in email with that use case.

There is a tracker bug here https://bugzilla.mozilla.org/show_bug.cgi?id=124287 every one of the depends on is a bug about weird results from certain characters, and some are mail server software dependent.

You more that welcome to file a bug for your issue, and you probably should, but the simple solution from my perspective is just don't use underscores to start folder names.

Thanks, your explanation makes sense, and I agree in a perfect world it should be fixed but I'm sure there are more important things to work on.

I checked the link you provided and I'll try to use characters that don't seem to cause problems, like space and hyphen to force my frequently used folders to the top of the list.

Overall I'm pretty happy with TB, much less quirky than my old Eudora, though I still use that for automated out-mail that I don't want clogging up my sent box.

Sort your folders how you want them without special characters. https://addons.mozilla.org/en-US/thunderbird/addon/manually-sort-folders/?src=collection&collection_id=35816d23-50da-46d4-b65f-7180f4c47dec

You do know you can specify in What folder outgoing mail is placed and even set it to save nothing for your mass mailings.

Tools menu (alt+T) > Account settings > copies and folders for the account.

I appreciate the suggestion and I am trying the add-on. Oops, how annoying. After you use the add-on to re-order all your folders just the way you want, if you ever rename one of those folders it loses its place and goes to the bottom of the list. Even if you re-name a folder that you did not previously re-order. Which means any time you re-name you have to re-order. I guess it's better than dealing with unexpected behavior.

To your second point, I do understand I can control where mail goes after it's been sent, but now I remember the real reason I use Eudora. It's because TB will not send the mail, or at least it wouldn't when I started using it. I forget what the protocol is called to allow another PC program to send email in the background via your email client, but my shipping/postage program uses it. It automatically sends an email to my customer when I print my shipping label, via my default email client, and does so without interrupting my work. TB did not have that capability. Does it now?