Is the email to that one recipient being returned as undeliverable? If not Thunderbird has done it's job and delivered the message to your providers SMTP server. It has no control after that.

Has this person checked their Spam folder to see if that is where it is going?

What happens if they send you a message and you reply to it? Do they get the reply?

No its not coming back as undelivered to me even if I have requested a delivery status notification. They checked the spam folder and its not in that folder. If I reply to their email they still don't received it.

I would guess they either have an edge device or a smart hosting arrangement with some company that is dropping the mail as spam for some reason.

Your mail provider will have server logs that will at least be able to identify the response their server got when it attempted to deliver the mail.

One thing to keep in mind is that the email protocols do not guarantee delivery, and many companies and ISPs rely on that in they spam policies.