Issue: You have an AD Synchronized (DirSync or Azure AD Connect) Office 365 environment and need to add additional email aliases to an Office 365 Mailbox. This could be in a situation where you have a generic address you’d like delivered to someone, email@example.com, or in the event an employee doesn’t use email and all their mail should be delivered to their assistants mailbox.
Application: This article applies to DirSync, Azure AD Connect or ADFS federated environments that are not running on-premise Exchange (“Hybrid Mode”) and therefore cannot use an on-premise Exchange to add email aliases
Solution: On a DirSync’d Office 365 environment, email address changes are managed in (on-premise) Active Directory. Note: this solution may also apply to Azure AD Connect, the successor to DirSync.
1) To avoid confusion, if any AD accounts currently have the email address assigned that you want to add as an alias to another Office 365 mailbox then you should remove it from those AD accounts.
2) Add the aliases to the destination AD account that you want the mail delivered to
Windows 2008R2 DC: Open Active Directory Users and Computers (ADUC), click view > advanced features, doubleclick the users account in the list of users (not through Find) and click the Attribute Editor tab
Pre 2008 R2 DC use ADSIedit:
Start > Run and type adsiedit.msc
Expand Default naming context
Right click the user you want to edit and click Properties.
3) Find the variable proxyAddresses
When you add new e-mail aliases, you need to make sure that your primary e-mail address is included and prefaced with with upper-case ‘SMTP:’. Your aliases, aka, secondary addresses should be prefaced with a lower-case ‘smtp:’. For example, I want my primary e-mail address to be firstname.lastname@example.org
In the proxyAddresses attribute use the following format:
to make an alias email@example.com I will specify lower case “smtp:” like this: “smtp:firstname.lastname@example.org”
Note: If you are troubleshooting an email address assignment issue you mail want to look at the ‘mail’ attribute as well.
4) To make the changes effective immediately, run a manual DIRSYNC using PowerShell on your DIRSYNC Server to synchronize the changes up the Office 365 cloud (by default automatic directory synchronization is every 2 hours).