The administration and maintenance of Microsoft Exchange Server to ensure secure, reliable, and efficient email and collaboration services across an organization.
Glad to see that your issue had already been resolved and thanks for sharing the solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others", I'll repost your solution in case you'd like to "Accept" the answer.
Exchange user mailbox delegation error 4003
Issue Symptom:
When trying to add send-as permission for this user to one of the probelm users, the following error is displayed.
Solution:
simply turning on the inheritance for that target user(s) did enable us to make the changes required in Exchange.
Best Regards,
Jarvis Sun