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27.3. Add or Remove E-mail Aliases in AD HYBRID

If you are synchronising your Office 365 account with your on-premises Active Directory environment, you will know that you cannot edit exchange user properties using the Office 365 administrator portal. If you try, you will come across this error or a similar one:

In this post, I’m going to show you how to add e-mail aliases using the Active Directory User and Computers snapin. Before we begin, make sure that you have the advanced options enabled from the view menu in ADUC. • With ADUC open, select view, and then check the box next to “Advanced Features”

Now, with that out of the way…

1. Select the user you wish to add an Alias for > Right click the name, and select properties.
2. Now, Remember that advanced features check box I mentioned above, because we selected that, you will now see a tab called, “Attribute Editor”. If you don’t see this tab, go back and check the box for “Advanced Features”, and then kick yourself, for not paying attention.
3. Find the variable proxyAddresses – this is the one you want to edit. When you add new e-mail aliases, you want to make sure that your primary e-mail address will start with upper-case SMTP. Your aliases, aka, secondary addresses should be lower-case smtp.For example, I want my primary e-mail address to be chrisisawesome@example.com In the proxyAddresses attribute, I would put: SMTP:chrisisawesome@example.com As my alias, I want chrisrox@example.com… to do this, I will use lower case smtp: smtp:chrisrox@example.com
4. Apply the new settings, and click OK
5. Wait patiently for your active directory to be synchronised with Office 365 (by default this happens every 3 hours but you can force this with a bit of research.

 

REF: https://cbudde.com/tutorials/add-or-remove-e-mail-aliases-in-on-premises-active-directory-office-365/