Nothing wrong on either end. That's simply a limitation of the whole process. Images may get arbitrarily resized or even re-compressed. Unless your client creates an explicit HTML template for this, it will always look crooked and even then since many users turn off HTML in e-mails, there is no guarantee it will actually ever show up. Anyway, as a start verify your own output in a "neutral" environment like a simple image viewer (not Preview). You may simply be using a specific color profile and/o r exporting at 300 DPI or something and naturally, since the web doesn't care for this stuff, it may look weird already outside your own computer. The rest - per above.
Mylenium