if your email is User1@Company1.Com and your alias is User1@Company2.net
1. Remove the Alias from User1@company1.com
2. Create an External Contact with Email User1@company2.net
3. Go to Manage My organization , Mail Control , Rules and then Create New Rule
4. On the Apply this rule if the recipient User1@company2.net , Do the Following redirect the message to User1@company1.com
5. Then Connect to the Office 365 PowerShell and Run the following Command “Add-RecipientPermission User1@company2.net -AccessRights SendAs -Trustee User1@company1.Com”
6. Test Send and receive emails from OWA and the drop list From will show both emails
7. To keep the From in OWA new message run the following Command “Set-MailboxMessageConfiguration -Identity User1@company1.com -AlwaysShowFrom $true
