I just received one of those e-mails today. I use Thunderbird for my e-mail client. I have had this happen in the past, not often, but it does happen and I have never been able to find out why.
The only thing I can guess at is the two e-mail clients involved are not compatible, or the content can not render in both clients for some reason.
I think I found the answer. It is the Thunderbird client in my case and the fix is in the last post on this forum:http://forums.mozillazine.org/viewtopic ... 5&start=15