You have a user or group that needs to receive email sent to multiple addresses.
This is essentially the same process as Add Email Alias (Secondary Email) to Office 365 Account that has AD DirSync, except that it can be done directly from ADUC instead of having to run ADSIEdit.
- Launch Active Directory Users and Computers
- From the “View” menu, make sure that “Advanced Features” is checked
- Open the properties of the user or group you need to edit and switch to the “Attribute Editor” tab
- Scroll through the list of attributes until you find the “proxyAddresses”
- Edit that attribute and add the additional addresses, prepending each with “smtp:”. Note that case is important here: the address that begins with “SMTP:” (all caps) is assumed to be the default address for that object (i.e. the address mail sent by that object will appear to come from), lower case “smtp:” is an alias.