sysutils/nut maintainer mail rejection

I've been trying to send email to the sysutils/nut port maintainer
(cy@FreeBSD) but the emails are bouncing at the very last stage with the
following type of message:

<Cy.SchubertXXXXXXXXX> <Cy.Schubert@cschubert.com>: host
XXXXXXXX.com[XXXXXXXXXX] said: 554
    5.7.1 <tom.b.rushworth@gmail.com> <tom.b.rushworth@gmail.com>:
Sender address rejected: Rejected (in
    reply to RCPT TO command)

(with specific details X'd out, although anyone who gets a similar bounce
 will have them).  This is making it past the FreeBSD mail server, it is
getting bounced at what looks like the very last step.

I don't think this is an email problem at my end (gmail _should_ have it
right :) ), can anyone
else confirm, or tell me what I'm doing wrong if it is at my end?

Thanks!
--
Tom Rushworth