Office 365 default E-mail address using ADSI Edit when using DirSync

Open ADSI Edit

Navigate to User

Right-click on Properties

Navigate to proxyAddresses or if Exchange Schema is loaded msExchShadowProxy

Add or correct   ->    SMTP: <email address is added> 


Apply the changes and force a DirSync to Office365 Azure AD

See the article: 

Force DirSync to sync with Office365 Azure AD