Assume that we have 2 Exchange organizations, one is responsible for *@MainCompany.com emails (MainOrg), the other is responsible for *@subCompany.com emails (SubOrg). Now we want MainOrg to receive emails on behalf of SubOrg, meaning all emails that are sent to *@subCompany.com address should go to Exchange server in MainOrg.
Note: SubOrg doesn't have to be Exchange, it could be any mail system
1. For all users in SubOrg, create contacts in MainOrg
2. Create a Recipient Policy that will generate exactly same @subCompany.com email addresses for contacts you created in step 1. This Recipient Policy should NOT be authoritative for subComapany.com
3. Change public MX record of subCompany.com so it now points to MainCompany Exchange server instead of subCompany Exchange server
4. Create a SMTP connector on MainOrg Exchange server, specify subCompany.com as its space
5. Enable "relay for this domain" on the connector created in step 3, forward all mail to subCompany exchange server (subOrg Exchange as smart host)
6. Restart Routing Engine and SMTP services
Caution: subOrg must be configured as "authoritative" for @subCompany.com