Hello we create Mailuser (or Mailcontacts) for people who need a internal Mailadress like (internaldomain.com) but use for example there own email account (gmail or whatever). The problem ist that the email will not resend like it is with a Mailbox where you set the forwarding. This result in SPF failure or softfailure. I know there is no SRS service for onprem. How you handle that in your environment.
We are running Exchange 2016 CU12.