Hi Gary. It is best to have an SPF, DKIM and DMARC records for each of your mail domains. This is the correct and most effective way to have your mail server set up properly. Your customers should not use the IP address of the SM server for their incoming and outgoing servers. Using the hostname is best practices. If you need some help setting up SPF, DKIM and DMARC for all of your domains, I have extensive experience with that and I will be happy to help you get started. Thanks.