You don't need multiple Exchange accounts.
This is easily accomplished through Recipient Policies.
Open Exchange System Manager
Expand 'Recipients'
Select 'Recipient Policies'
In the right pane, right-click 'Default Policy', Select 'Properties'
Select tab 'E-Mail Addresses (Policy)'
Here is where you tell the server what email domain names it is responsible for. The name of your existing domain is already there.
Click 'New'
Select 'SMTP Address'
Click 'OK'
In the address box, type in the domain name you want to add, using the following format:
@abcdomain.com
Click 'OK'
Your additional domain name should now be listed.
Click 'OK'
Right-click 'Default Policy' again, select 'Apply this policy now'
The server is now configured to receive emails under various domain names.
For example...
User1's email address is both
user1@abcdomain.com & user1@xyzdomain.com
using only one exchange account.
The next step is to set individual user preferences (optional)
Open Active Directory Users and Computers
Open up a user account
Select tab 'E-Mail Addresses'
You should see all available email addresses.
From here you can select which is their primary email account. This is the email address that is used for all outgoing mail.
Don't forget to properly configure your MX records in your public DNS.
This method has worked flawlessly for me.
I have 1 Exchange server, 150 users in 9 different subsidiary companies and 14 domain names.
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