Some recipients get a jumbled newsletter

I am using the latest version of Zen. I send newsletter every week and most people do get it properly. However, a few customers mentioned that they cannot read the newsletter. One person did send me what she got and sure enough, it is like all overlapping: the header, the text, the title, etc. So it IS unreadable.

Any explanation why some customers do get it jumbled up? They were set for HTML format of the newsletter. Should i change them manually to text format? This week, i happened to send an email to my subscribers, and even though i sent it with rtf (or html which ever), one person NOT receiving readable newsletter, got this email fine (that is how she was able to notify me that she was not receiving the newsletter).

Any suggestion or explanation?