Hi @Tony-CMM 

There was no screenshot attached but the fact that the email looks fine in some email clients (you mention Outlook app / iphone mail) suggests it is something about Outlook Desktop rather than the email itself.

As you will know from your experience in digital marketing, there have always been issues with the way different browsers (& this extends to email clients) render html code.  Some browsers simply don't follow the rules in quite the same way - or aren't so forgiving of coding errors & foibles.

I don't use either QB emailing system or Outlook Desktop so can't delve into it.

Have you thought about switching to plain text emails with pdf attachment?

Or downloading the pdf invoice & emailing outside QB?  (that's my preferred option as I need to customise the message in a way that's not possible otherwise).