Where does "my copy of the email" come from? Do you Cc yourself?

Zenos said

Where does "my copy of the email" come from? Do you Cc yourself?

Yes, I copy myself but also the fonts are different in the Sent folder.

There are separate settings in Thunderbird for the fonts used to display a message and those used or declared when composing. In fact, ordinarily, if you inspect the source of a message you sent, it usually doesn't actually specify the typeface; it leaves this to the recipient to specify how email should be displayed. I usually find that my messages, composed using a sans-serif presentation when I write them are quoted back to me using a serif typeface by people who use Outlook. The default font in Outlook is often Times Roman, so that's what it assumes if Thunderbird doesn't explicitly set a typeface. Outlook also messes with the size.

I can't think of a definite reason for what you describe. All the same, I'd look at the settings for the fonts.

http://xenos-email-notes.simplesite.com/417754775

I'd look specifically at the different fonts used under different encodings. Saved messages are sometimes converted to an unexpected encoding and that might be where a different font creeps into your local copies.

If you want an explicit typeface to be used in outgoing messages, a Stationery template might be useful. You would at least have some control over the default font typeface and size, though still little or no influence over the recipient's formatting choices. And bear in mind that not everyone else will have your favourite font on their computer.

https://addons.mozilla.org/en-GB/thunderbird/addon/stationery/