Gmail delivery sometimes fails to domain1 but never to domain2, both domains have the same MX, etc

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http://serverfault.com – Lets say we have 2 domains, domain1.co.za and domain2.co.za. Both point to the same MX records with the same priority, mx1.isp.co.za and mx2.isp.co.za, both of which send the mail through our ISP's filtering/scanning service. From there, mail for both domains is delivered to our on-premises Exchange server using SMTP. In other words, the routing for messages sent to both domains is exactly the same. We are seeing consistent failed delivery of messages sent from Gmail to domain1.co.za, but only for some Gmail users. For some Gmail users, messages are consistently delivered to domain1.co.za. O (HowTos)