Using Outlook for iOS in business environments for email and scheduling
I'm having precisely the same issue. From what I can tell, every email I send gets converted to 1.5-line spacing and a new font (Helvetica Neue instead of Calibri).
I would love to make Outlook my default email app on iOS, but I can't do it if it my sent emails get changed this way. I just want my sent emails to look the same as if I had sent them from my desktop Outlook app.