For the WebApps it is present in the various server.xml's in the Web/WebAppServer*/conf directories as well as in some metadata configuration files.
https://go.documentation.sas.com/?docsetId=bimtag&docsetTarget=n1fpiezp819c6mn1iy2aq5q9oau2.htm&docsetVersion=9.4&locale=en
You might consider running the SDM changehost to update the references - if the mailhost was a unique host and you can blindly replace the old name with the new name anywhere it's found.
https://go.documentation.sas.com/?docsetId=bisag&docsetTarget=p1gs13ueroyethn11y3tpku021sd.htm&docsetVersion=9.4&locale=en
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