My Thunderbird does exactly what you're asking for. How does yours fall to satisfy this description?

Zenos said

My Thunderbird does exactly what you're asking for. How does yours fall to satisfy this description?

Sorry, I'm using Daily. Maybe that's different. Apparently the default value for "Send Options" is to send plain text because I don't think I've ever changed it myself. So with it set to plain text by default I've composed a couple messages and formatted the text using the menu--not realizing the formatting I was doing would not be preserved--and sent the messages only to find out later that only the text was sent without the formatting. If it is only going to send plain text then formatting the message should not even be an option.

There are two things going on here.

Clearly you are not creating a plain text email as you can see and use the 'Formatting Bar'. This is determined by your selection in account Settings for that mail account. right click on mail account name in left Folder Pane and select 'Settings' select 'Composition & addressing' for mail account. select: 'Compose messages in HTML' - (if you want HTML) click on OK

But that has no baring on how you choose to send an email. Menu icon > Options > Options > Composition > 'General' tab click on 'Send Options' button 'select the option: 'Send messages in both Plain Text and HTML' click on all OKs It is usual to select to send in both HTML and Plain Text, thus allowing the option on how to read up to the recipient.

You can also set up individual contacts to receive in eg: Plain Text. Again this has no effect on how you composed the email, but it will effect how that recipient will see it. This is done in the Address Book. select a contact either double click to open Properties or click on 'Properties'. 'Prefers to receive messages formatted as 'Plain Text'

You can also choose to swap to plain text for composing by holding down 'Shift' and clicking on 'Write', but it will only be for that email.

So as you can see there are a number of combinations going on and it depends upon what settings are selected.

Composing in HTML is based on a per mail account setting, which is only created after adding mail accounts, so is obviously not a default setting. You could have one set up to compose in HTML and another set up for Plain Text. So it would depend upon which mail account was selected and viewed prior to clicking on Write. So there are a number of combinations.

Sending is a global setting used by all mail accounts.

How emails are sent does not effect how they are composed, only how they are sent. But I can your point if only sending in Plain text was preselected by default.

However, I do not recall it being set as default when I originally downloaded, so that default setting in itself is unusual.