Mail: All email to a domain to one user

I am trying to work out how to configure a user's account in Server 5 on El Capitan to be the recipient for all email for @example.com.

In Yosemite, I used Workgroup manager and configured an email address of @example.com to the user's profile, and this worked fine.

As Workgroup Manager does not seem to be available for El Capitan, and the user configuration panel within Server.app seems to require a user account prefix to an email address, I am struggling to find a way to do this.

Has anyone else faced — even better, solved — this problem?

(I seem to recall that editing a config file might have done the trick previously; I would prefer something "official" if possible, to minimise the chance of the configuration being overwritten accidentally.)