I'm having the same issue with Helvetica/Arial in Outlook 365 on my iMac. More often than not, instead of text being invisible the characters will appear as boxes with question marks in them. It only affects text that is formatted to be Helvetica, Arial, or Sans-Serif with normal styling (bold and italic are not affected). The problem only affects emails I receive in Outlook as far as I can tell - if I open an HTML email in my web browser the text shows up fine. Thanks to another suggestion in this thread I discovered the workaround that I can read the garbled text by clicking the reply button. Kind of a pain, but at least it allows me to function. I really hope Apple fixes this soon.