“This email address cannot be used” error

Why do I get the error “This email address cannot be used” while saving a delivery server settings?

Starting with version 1.9.23, the email address that you use in the "FROM" field of a delivery server must be unique across all the customers in the system.
What this means exactly, if you have a customer A that uses a delivery server which has a "FROM" address set to "a@a.com", then customer B will not be able to create a delivery server and use same email address.