I work for an org that controls the main domain name. For sake of example we will call it cookie.com.
If you wanted to email me you would just sent me an email at email@example.com. Now the org I work for only gives you a POP/IMAP account with 500MB quota, so I run an exchange server for my user group so they have have more things than just plan email and a low quota.
For sake of example the exchange domain name is exchange.cookie.com. So if you wanted to email me you could email me at firstname.lastname@example.org. The organization give a user the ability to forward their email to anywhere they want. So I can give people email@example.com and it will automatically forward to firstname.lastname@example.org. Now some people have multiple email addresses setup in their forwards such as a yahoo or gmail account.
Here comes the tricky part. I've figured out how to have Exchange 2007 change the email address to be from email@example.com vs firstname.lastname@example.org. The problem is if someone else that uses the exchange server replies to that person, exchange routes it internally so it never hits the orgs forwarder that will allow the email to be forwarded to someone's gmail account.
How do I get Exchange to send email sent to email@example.com to go to the SMTP smart host versus internal routing?