Outlook, indeed, produces some artifacts.
E.g., you might see a lonely "J" that appears where a smiley was put by the author:
http://chris.pirillo.com/2010/06/25/j-smiley-outlook-email-problem-and-fix/And MS doesn't consider that being a problem:
https://www.buzzfeed.com/kasiagalazka/smiley-faces-js?utm_term=.aooypnLrZ#.ug1AQ3rRdAs for the specific problem, I suspect that it might be due to the difference between the end of line symbols on different OSes and different e-mail clients (^M vs ^J vs ^M^J aka CR vs LF vs CR LF), and how that is parsed by Aquamail (or WebView component on Android). I would suspect that Outlook might use ^M^J:
http://ergoemacs.org/emacs/emacs_line_ending_char.htmlThere could be also some additional effect from how the text is sent by Outlook (Format=Flowed vs. not), especially if it is a plain-text message.
Or, if it is an HTML-formatted message, the Outlook HTML idiosyncrasies could be the culprit, as pointed out by @KieSeyHow.
If you look at the "source" ("view source") of the e-mail message (say via the webinterface of your e-mail provider), you should be able to see how the end of lines are implemented in the offending messages.