my guess is you have set some recipients or domains as text and are sending them text not HTML mail.

An idea, Matt, thank you. But I have not consciously set any recipients or domains as text. Checking two emails, one of which gives the problem and one which does not, shows that both addresses are set to the 'Prefers to receive messages formatted as:' default Unknown.

Also check

Tools menu (alt+T) > options > Composition > Send options. Set it to HTML only, there are very very few requirements for plain text in email these days.

That has not fixed the problem with existing Sent emails, but maybe it will for the future.

if they are text, then that is not going to change. However changing the font I think will force the mail into HTML format, you just have to do it from the format menu rather than a toolbar.

Or use the options menu when you have the message open to edit to specify the delivery format as rich text (HTML)

I cannot change the font because the Format menu is not showing and specifying HTML with the message in edit mode does not work either. There does seem to be a bug in TB.

Were the original messages sent with HTML formatting? Or maybe they did not do any explicit formatting that required HTML, so rolled back to plain text? Ctrl+u on a sent message will show the source and reveal if there is any HTML code present.

To force the issue, you could select all the text in a sent message (ctrl+a) and paste it (ctrl+v) into a new HTML message.

Zenos, all my messages were sent with the TB default settings and there is no obvious difference between the different sent messages. Following the earlier discussion on this thread, I have now set messages to be definitely HTML, so this problem may not occur in the future. Your explanation of the reason for the problem may be correct, but it is still annoying and unexpected. I can certainly copy and paste text from a sent message to a new message, but that spoils the benefit of 'Edit as New' because the recipients, subject and any attachments have to be transferred too.

What's the use case that stops you using forward or reply? Edit As New is a somewhat quirky way to generate a new message. Or maybe a template or draft would be a useful starting point, instead of tweaking an old message? Or a Mailing List?

My experiments with Edit As New show it using the mode appropriate to the content; if the message has HTML content, I get the relevant toolbars. If it was plain text, then I get the plain text edit mode. And there's no easy way to convert a plain text message to HTML except by copy-and-paste. So I'd expect the messages that open without formatting options will be plain text. Matt's suggestion should stop this happening again, if you can force HTML by default for all new messages, but I don't know of any way to "repair" these old ones.

The QuoteAndComposeManager add-on has an option to force HTML even when Thunderbird might de-select it if there's no content that requires HTML.

https://nic-nac-project.org/~kaosmos/realborders-en.html

What's the use case that stops you using forward or reply? Edit As New is a somewhat quirky way to generate a new message. Or maybe a template or draft would be a useful starting point, instead of tweaking an old message? Or a Mailing List?

Edit As New saves a lot of time if one wants to correct/amend a previously sent message. The other options, all of which I use, take more time to get the same result with the possibility of errors.

As you say, Zenos, There does not seem to be a way to "rep[air" old messages but I now hope, with the Send option changes I have made, that this problem will not occur again.

Thank you and Matt for your help. I'll close this thread now.