there are several threads explaining how to do this in this forum, although the solution that you are looking might be to configure a connector in exchange which can route mails for email@example.com to your Service Desk server.
Here are the steps to configure a connector in exchange:
1. Open Exchange System Manager (ESM) console and navigate to the Connectors Container. Note: If you can't see the Administrative Groups you have to go into the properties of the Exchange Organization object and enable the view of Administrative Groups and Routing Groups.
2. In the Context Menu of the Connector Container click New SMTP Connector. 3. Give a name (ex: Exchange to Servicedesk) 4. Select "Use DNS to route to each address space on this connector". 5. Select "Forward all mail through this connector to the following smart hosts" and enter service desk server's FQDN (i.e. sd.mydomain.com). Note: If you want to specify IP then IP address of the Smart Host must be specified in Brackets (this prevents Exchange from trying to resolve the IP Address with DNS). 6. In Address Space for the SMTP Connector select address Space type as SMTP and in Address field specify Service desk server's FQDN i.e. sd.mydomain.com 7. In Connector scope select "routing group" for connector scope, and check "Allow messages to be relayed to these domains". 8. In content restriction page uncheck System Messages.
Then create in the Active Directory a CONTACT that has the address firstname.lastname@example.org AND email@example.com with the second one set as the primary SMTP address.
Exchange will then accept email to firstname.lastname@example.org and will forward them to email@example.com
Joseph, Give some points to RandallB as he offered almoast complete instruction but the one thing - exchange contact has only one address. So you can choose the contact from addressbook and send e-mail directly to SD server. If you want to route e-mail from internet to SD, so you have to create some account with firstname.lastname@example.org address and in its exchange properties configure Forward all mail to [Created earlier contact or address email@example.com]. Exchange connector will do its routing function. As another way to route mail - you can add MX record for sd.domain.com dns zone pointing to your sd server (do it on DNS server that is used by your exchange)