You don't have to do it that way. There is a much easier and secure way to to it.
Unfortunately, I am not at the office right now. If I were at the office, I could visit the server and give you step-by-step instructions. But I'll try to point you in the right direction off the top of my head. Maybe someone else can be more descriptive.
Here goes:
In Exchange Recipients, go to New --> Other Recipient (?). You then enter the SMTP address of the person in question. This would be his/her actual outside email address. Then, you go to Email Addresses and ADD a new Internet address for that account. This would be the "name@yourdomain.com."
Your company Address Book will show up the person's Name unless you Hide it in the advanced tab. Once you have done this, you should notice the Recipient Icon looks like a "globe" instead of like the rest.
Exchange will redirect any incoming email with the address of "outsideperson@yourdomain.com" and redirect (forward) it to "actualperson_address@their_domain.com.
Doing it that way eliminates the need to set up an account and password for that person. You don't have to set up anything else on Exchange such as relaying either.
Hope this leads you to where you need to go. If I can find a related procedure, I will post it below.