[exim] Mail to exim-users failed in IPv6

Well, quite surprising is this:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  exim-users@???
    SMTP error from remote mail server after end of data:
    host sesame.csx.cam.ac.uk [2001:630:200:8080:20e:cff:fe09:a58a]:
    550 Administrative prohibition

Don't you support IPv6 connectivity but advertise it?
I hope this one will make it through.