SharePoint sends outgoing email as anonymous SMTP. Under Exchange 2003 this wasn’t much of an issue as all you had to do was allow the SharePoint server IP as an allowed relay under the SMTP virtual server. In Exchange 2007, however it becomes a bit trickier since Exchange 2007 has built-in security to make it more difficult to create open relays.
First you have to create a custom receive connector with the IP addresses of your SharePoint front ends listed as allowed relays. Set your authentication to TLS and Externally Secured. Set your Permission Groups to Exchange Servers and Anonymous Users. Lastly you have to allow the Anonymous Login extended rights as an Authoritative Domain Sender on that particular receive connector.
Create the new Custom Receive Connector, I named mine SharePoint.
Enter the IP Addresses of all your SharePoint Front End servers
Hit new to create the new receive connector. 
Now you have to edit the newly created receive connector to change the Authentication and Permission of the receive connector.
Change the Permission Groups to Exchange Servers and Anonymous Users
Change the Authentication to Externally Secured
Lastly, you have to extend Anonymous Login Domain authoritative sender rights using Power Shell.
[PS] c:\>add-adpermission 'sharepoint' -User 'NT AUTHORITY\ANONYMOUS LOGON'
-ExtendedRights ms-Exch-SMTP-Accept-Authoritative-Domain-Sender |