Azure/Office 365 – Usage of proxyaddress attribute and limitations

ProxyAddress attribute is used by different applications and it can store different type of user addresses (sip, smtp, x500).

If you sync accounts with non-verified domain to O365, those addresses can be replaced with the default onmicrosof.com domain.

Some interesting reading regarding this topic:

ADConnect not Syncing ProxyAccount for email Alias from on Premise AD to Azure AD

https://social.msdn.microsoft.com/Forums/azure/en-US/3226e41c-1db1-4299-9f24-0179e05fac09/adconnect-not-syncing-proxyaccount-for-email-alias-from-on-premise-ad-to-azure-ad-i-am-using?forum=WindowsAzureAD

List of attributes that are synced by the Azure Active Directory Sync Tool

https://social.technet.microsoft.com/wiki/contents/articles/19901.dirsync-list-of-attributes-that-are-synced-by-the-azure-active-directory-sync-tool.aspx

A mail user who has proxy addresses that use non-verified domains isn’t synced in an Exchange hybrid deployment

https://support.microsoft.com/en-us/help/3124148/a-mail-user-who-has-proxy-addresses-that-use-non-verified-domains-isn

Wrong domain address when synchronizing from on premise AD

https://social.msdn.microsoft.com/Forums/azure/en-US/7ddc1885-850d-487f-bf40-a91f1f5d15c8/wrong-domain-address-when-synchronizing-from-on-premise-ad?forum=WindowsAzureAD

Azure AD Connect sync: Attributes synchronized to Azure Active Directory

https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnectsync-attributes-synchronized