External meeting requests cannot be forwarded to internal recipients because Office 365 Exchange sees the message as coming FROM the original external sender, not the internal sender actually sending the message. We use a 3rd party email filtering service and have setup a Rule in Office 365 Exchange that prohibits the delivery of any email that does not come "FROM" the IP address of the 3rd party filter. The message will fail with a 5.7.1 error.
Does anyone know of a way to alter the RULE in Office 365 Exchange so that it will make an exception when an internal user is forwarding a meeting request that originally came from an external sender? I know this may be confusing. Please let me know if you need any clarification.