I have two users who, at times, when they email each other through their mobile devices through their work email, the reply shows up in this very light yellow font. At first glance, it just looks like a blank reply, but if you highlight it, then copy and paste it to say, notepad, you can see the written words.
It only happens between these two individuals, and it's not every time either. If they email other people in the company, their emails show up fine, so I don't think it's a font setting (but I could be wrong).
We run an exchange server 07 here and most people have their phones connected to email.