If one has one’s LAN Active Directory synchronized with EOL/Azure, one cannot add secondary email addresses in the EOL console. In this situation:
- Open ADSIedit from the domain controller
- Open up the OU containing the user
- Open the Properties of the user
- Open the Properties for the item “proxyAddresses”.
- The primary (the “reply”) email address for the user needs to be specified thus, with caps in the prefix:
SMTP:user@domain.xyz - Secondary email addresses for the user need to be specified thus, with lowercase prefix:
smtp:alias@domain.xyz - Then run the sync or wait for the automatic run, and it’s done!
Also, as a bonus, after the above is done once, user objects in Active Directory Users and Computers get a new tab, “Attributes”, from which the above can be done for other users.