Using Outlook for iOS in business environments for email and scheduling
I don't see it noted elsewhere, but in case it's helpful to anybody we have found this issue applies when sending from iOS Outlook app and reading in Windows Outlook (365 in our case). Sending or reading with other applications seems OK, and opening the received source code in Word exhibits this same issue.
The sent mail on my iPhone looks OK without any of the Helvetica font or 1.5 line spacing. And even users who read the *first* email on their iPhone or Android devices do not see this issue (the first time)
But if someone from Desktop Outlook (Windows & Mac) or even Outlook Web replies to that mail, their reply content is fine but the original email displays with the font & line spacing issue for all users - desktop & mobile alike
to put it succinctly, any involvement of desktop Outlook s***ws up the whole email :(