Create mail forwarding to an external email alias
User has an AD account in one domain with a particular email address. That address needs to be forward emails to another external email alias.
To resolve:
Create a mail user in Exchange 2010. Use the pshell one liner below to accomplish this
Enable-MailUser -Identity jdoe -Alias 'jdoe' -ExternalEmailAddress 'SMTP:alice@blahblah.org'