[Exim] A variation on 'Null envelope sender not allowed'

G'morning.

To make a change from broken mailers replying with

550 <>: Sender address rejected: Null envelope sender not allowed

I have got the following

2003-03-07 00:06:22 18qu77-0007Pm-00 ** some.one@???
<Some.One@???> R=lookuphost T=remote_smtp: SMTP error from remote
mailer after end of data: host inbound.registeredsite.com [64.224.219.122]:
554 5.7.1 Non-bounce message missing envelope FROM address!
2003-03-07 00:06:22 18qu77-0007Pm-00 Frozen (delivery error message)

This is in response to a vacation message sent out by a Cyrus/Sieve combo.
The message does indeed have an envelope sender of <> and a normal From:
header.

Can someone better versed in the legalities of these things confirm my
belief that the remote end is severely brain-damaged?

TIA,
Richard Hall