Blocking Direct Delivery Email for Specific Domain

[amandler]

One of my domains uses a third party spam filtering service, which is defined in their MX record. However, they still receive many spam messages by "direct delivery" where the message sender has bypassed the MX record and sent directly to the host server, avoiding filtering.

Is there a way to allow only mail relayed from the 3rd party filtering service to reach the domain, either through Postfix or the Plesk Spam filtering settings?

Thanks.