How to manage SMTP addresses for mailbox users.
Security Explorer provides management of SMTP addresses for contacts, mail users, and distribution groups. With version 9.6, mailbox users are also included. You can add many email addresses, but only one email address is designated as the primary address.
Within the Security Explorer console, select the Exchange Security module.
Expand the Domain.
Expand "Mailboxes (All)".
Browse to and right click on the mailbox of the user you would like to create a new SMTP email address for.
Click "Delivery Option"
Type in a new email address.
Click "OK" twice to finish.