You can use Internet Mail Service to relay mail for multiple Simple Mail Transfer Protocol (SMTP) domains and their associated subdomains to a specific smart host.
This option is made easier by the default routing property of Internet Mail Service to accept all mail for a domain unless the domain is explicitly designated by using a number sign (#) before the main domain. For example, an inbound domain of #microsoft.com only accepts mail for the microsoft.com domain, whereas microsoft.com accepts mail for all domains in the namespace of microsoft.com.
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