email signature from my MacBook Mail account is saved as an .htm file

My Windows 11, MS outlook,  does not display an email to Clients, that  I have cc'd myself in the same manner I have sent it?

eg. My email signature arrives as an .htm file?...and it sits in the ''attachment'' bar....and the font changes...to some sort of plain (non descript)  text?.....so my ''presentation email/s''...looks pretty awful?

When viewing the same email on my MacBook Mail...it is exactly as I have sent it?

Please help...as I'm sure MS Outlook needs an ''upgrade''?...or is my setup incorrect?

Kind Regards

Kevin R