I suspect they compose in Word or similar and then paste into Thunderbird. Something like that bent arrow symbol appears in Word when you use shift+return, though it's not clear why your contributors would be using it; maybe force of habit, or they don't know any better. ;-)

Shift+return is a handy way to force a line end without creating a paragraph break. Useful in bullet point lists to create multiple paragraphs, or organize pictures. Or they're using something like an Apple which uses non-standard line breaks.

I don't think Thunderbird's editing tools are capable of doing the search-and-replace that you have asked about.

If you want to do all your editing inside Thunderbird, you may be able to copy then use "paste without formatting" (ctrl+shift+v) to eliminate these line breaks. Doing this will also remove other formatting, so may create more problems than it solves.

I think in your position I'd take the text away to a better-than-average editor where a proper search-and-replace could be done, or you could select the lines and join them. Notepad++ would be my choice on Windows, probably Bluefish on Linux.

And a proper word processor may also be able to tidy it up, but the challenge then would be to avoid importing all the formatting overheads that would come with it when copy-and-pasted from the word processor into Thunderbird. Again, Thunderbird's "paste without formatting" might be useful here too.

I found out it was ascii 11 decimal. Turns out I can replace it in Word.

I several contributors who can't even send an email. I don't try to train them.

Thanks for responding.