Outlook isn't the only email client out there (I know, I can hardly believe it myself). So there exists an Internet standard - called iCalendar - for sharing appointment/calendar information with others. (FYI - this is the standard used by Apple.)
If you aren't sure what calendaring program your recipient is using you can simply send them an iCal entry right from Outlook, its easy:
- Open the appointment you want to send
- Go to Actions | Forward as iCalendar
- Address and send
The email will be received with the calendar entry as an attachment with an extension of .ics, the recipient will be able to open it and add it right to their calendar. Many common, popular and new calendar clients support this standard. You can read more about the iCal standard on Wikipedia.
Keep in mind that as you work to maintain positive relationships with your clients, one simple and useful thing you can do is collect a little information about what technology they use by way of Technology Compatibility Survey. Read this old post for more and a sample survey.
