[Exim] ETRN

I have been asked by two clients to provide mail for M$Exchange.

Having looked through the archives and found a few references
to ETRN I 'think' I have set it up correctly but still get the failure

"458 Administrative prohibition" or simply
"Rejected ETRN" and that's it.

I have:
smtp_etrn_hosts = the.host.net
smtp_etrn_command = "/usr/lib/sendmail -R ${if match
{$domain}{^[@#]}{${substr_1:$domain}}{$domain}}"
smtp_etrn_serialize = false

in my MX records I have

the.host.net IN MX 10 smtp.the.host.net
the.host.net IN MX 20 smtp.my.host.net

So what am I doing wrong and how do I fix it?

Any help would be appreciated.

Regards

Ken