Block Messages to Exchange Group Except From Certain Domains

Let's say you have a group called "Everyone". But you only want internal people to be able to email the group and possibly another company.

• -open the EXCHANGE MANAGEMENT SHELL.
• -type: set-TransportRule -Name 'Block Email to GROUPNAME Except from ACCEPTED Domains' -AnyOfToHeader This email address is being protected from spambots. You need JavaScript enabled to view it.' -IsValid true -Actions {RejectMessage} -ExceptIfSenderDomainIs {validDomain1.com, validDomain2.com}

There are some other parameters in there too but that should do it.

If you want to do it visually:

• -open the EAC.
• -click MAIL-FLOW (on the left-hand side).
• -click NEW.
• -type: A-NAME-FOR-THE-RULE
• -click THE MESSAGE > THE TO BOX CONTAINS.
• -search for GROUP-NAME.
• -click ADD > OK.
• -BLOCK THE MESSAGE > REJECT THE MESSAGE AND INCLUDE EXPLANATION.
• -type UNKNOWN USER or some other explanation.
• -click MORE OPTIONS.
• -click ADD EXCEPTION.
• -click THE SENDER > DOMAIN IS
• -type: domain1.com
• -click +
• -and so on.
• -click OK > SAVE (at the bottom).