So you need to add an email alias to an existing Office 365 account that is sync’d with an on prem AD. However, you go into the user account in the 365 portal to find the following challenge:

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Unfortunately to do this you must use ADSIEDIT.

On a local domain controller open ADSIEDI.  Expand Default naming context Expand DC=yourdomain DC=yoursuffix

Navigate to the correct OU where the user is located Right click the user you want to edit and click Properties.

Scroll down to the attribute: ProxyAddresses and click Edit.

Add a new entry with smtp:yournewemail@domain.com (Ensuring its a lower case smtp as that denotes an alias)