Mac mail (version 6.2) text doesn't show in windows outlook

Hi there!
Some people using Windows can't read the e-mail text I'm sending, but they can read the e-mail text on their Blackberry.
What must I change in my settings to reach all operating systems?
Soonest advice is much appreciated!
Many thanks,
Nina

Check your Mail preferences to see what you're using for composing the messages. There should be two choices: Rich Text and Plain Text. Both should be readable by anyone using Outlook for Mail. If they can read the mail on their Blackberry, then the problem is likely on their end when using Windows Outlook.

You can try switching the compose format as indicated above, but I'm not sure it's going to help. Also, the latest version of Mac Mail is 7.0.