Outlook downloading mail with just an internet connection

Question

we are in an exchange 2019 hybrid setup with all our mailboxes stored in the cloud

We recently discovered that domain joined laptops running Microsoft 365 Outlook program are having their emails download/sync with just an internet connection i.e. with no Corporate LAN connectivity or VPN.

I believe it has something to do with with the Direct Connect feature but we have put in the suggested reg key 'ExcludeExplicitO365Endpoint' but the Outlook client is downloading mail. Am i missing something? Is there any other way to stop mail from downloading via outlook on domain joined laptops when they are not connected to our network? Would it need a conditional access policy?

Answer 1

Hi @NP,

From the perspective of Exchange side, if client access rules hasn't been disabled in your tenant, you can create a client access rule to restrict user's Outlook access based on IP addresses:

New-ClientAccessRule -Name "RestrictOutlookAccess" -Action DenyAccess -AnyOfProtocols OutlookAnywhere -ExceptAnyOfClientIPAddressesOrRanges <AllowedIPRanges>

It's likely that it can also be realized using Conditional Access policies, so I'll add the tag as well so community members over there can also have a look at it.

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