If you have a Mac with Microsoft Office installed, I'm going to take a wild guess and assume that you're using Entourage, a part of the Mac Office suite, for your email. That being the case, it turns out that there are some important preferences that you probably need to adjust to sidestep your reported problem:

You'll want to launch Entourage, then go to the Entourage menu item and choose Preferences, then click on Compose. Here's what you'll see:

The key value here is to ensure that you've selected Windows as the "Encode for" preference, as I have in my configuration. It's yet another area where us Mac users have to tweak things to meet the needs of our, um, platform-challenged Windows colleagues. :-)

I think that'll fix your problem. If you're running Apple Mail or a similar application, there are equivalent configuration settings, again, just dig around in the Preferences a bit.