I have been in a chain email dating back to July 6, 2013. On September 4, 2013 I started receiving unformatted emails from two of the 11 people on the email. I noticed that both of them are using Gmail and an iPhone. However, almost every other person on the email chain is using Gmail and an iPhone. Then I noticed that one person has sent an email that rendered fine, the next day sent one that didn't not render correctly, and then the next day sent another that rendered fine. As of today, every email that is part of the chain has come unformatted.

What I mean by unformatted is that the body of the email contains parts of the email header and then both text and html content types as plain text. 

The only thing I have noticed in the header is the X-Message-Statues is "sF:n" when it is unformatted and "n:n" when it renders correctly.

Why is this happening?