What mail provider? Outlook for one strips what Thunderbird sets and replaces it with what is registered on their web site.

Microsoft Live Hotmail smtp.live.com is the mail provider.

Everything was working normally, i.e. displaying my name as the sender, until recently. I don't remember when I updated to this latest version of Thunderbird.

When I send a message to myself: I look in my sent file: my display name is there as the sender and as the recipient When I look in my inbox for this same message, my display name is NOT there as either the sender or recipient.

Thanks

log in using your web browser, edit your profile on Outlook.com to show the display name you want to use for mail.

Then send a mail from Thunderbird. Not the change in information on the received mail.

This solved the problem. Still don't know how the account display names (more than 1 account involved) got changed. I didn't change them. The email provider probably uttered those infamous words: "It's just a small software change / upgrade". Thank you Matt for your help.