Mail not sending but works fine in Thunderbird.

I know this has been asked before but I did not find an answer. I am using a MacBook pro running 10.15.7. My mail has stopped sending and, although I can see it on my host in the sent mail, it does not reach the destination. If I use Thunderbird with the same settings it also gets to the host but it always reaches the destination. I cannot find any difference with the SMTP/IMAP settings. I would upgrade the OS to Big Sur to see if it helps but it's not an option from the early 2013 it seems. Does the Mac insert something in the email header that would cause this. As mentioned, TB works fine but I prefer the built in mail app.

Thanks in advance.

M.