Difference between `proxyAdresses` and `mail` attributes in Active Directory

As you said, the proxy address attribute can contain multiple values whereas the mail address contains only a single value. This can be useful when configuring multiple addresses for a single account. See the below config:

proxyAddresses: SMTP:[email protected]

proxyAddresses: smtp:[email protected]

proxyAddresses: smtp:[email protected]

In this instance, the first attribute "SMTP:[email protected]", being uppercase, defines the user's primary email address. And will also be the default address for Outlook and Outlook online.

There are 3 attributes that need to be configured to ensure Accounts are synced properly between your on-premise domain controller and AzureAD/Exchange Online. These are mail, mailNick and proxyAddress.

An example of a working configuration would be as follows:

mail: [email protected]

mailNick: John Smith

proxyAddress: SMTP:[email protected]

proxyAddress: smtp:[email protected]

proxyAddress: smtp:[email protected]

From what I know the mail: attribute is more a contact attribute as it can exist without Exchange against a user.

The proxyAddressss are the ones used to deliver mail primarily used by exchange.

Give a look at this Url:

https://techcommunity.microsoft.com/t5/exchange-team-blog/fun-with-changing-e-mail-addresses/ba-p/609781

Fun with changing E-Mail Addresses – You Had Me At EHLO…