If you setup a Forward for a user to an external user , and a external party email’s that user , then this counts as external to external which is an external Relay which is not allowed most of the time due to spam.

In order f0r this to work in Mimecast you would need to add the external recipient in this case ‘[email protected]’ to the ‘Relay’ profile group in administration> directories> profile groups, as the below default blocked sender policy is set to ‘take no action’ on this group and allows an external sender to send to an external recipient as long as the recipient is in this relay profile group.