Hi Brad,
Do you mean that you want to give the user named admin@abc.com Send As permission for user1@xyz.com?
If yes, we cannot use the proxy address to send emails, so we cannot use admin@abc.com to send emails as user1@xyz.com. Therefore, your requirement cannot meet your requirement.
If you want to send emails with user1@xyz.com, you can try the following steps:
1. Remove the proxy address (user1@xyz.com), create a distribution group (user1@xyz.com).
Create a New Distribution Group: http://help.outlook.com/en-US/140/ms.exch.ecp.newdistributiongroup.aspx
2. Give the user named admin@abc.com Send As permission for user1@xyz.com, admin@abc.com can send emails as user1@xyz.com. For detailed information, please refer to the following link: http://help.outlook.com/en-us/140/ff852815.aspx
3. You can create rules to forward or redirect all the emails that send to user1@xyz.com to admin@abc.com. For more detailed information, please refer to the following link: http://help.outlook.com/en-US/140/ms.exch.ecp.NewTransportRule.aspx
Thanks,
Anna Shi