[Exim] 451-Invalid HELO

Dear list,

I've been sending emails to *.ox.ac.uk for years, a few weeks ago all my emails started getting
delayed, and then eventually cancelled, with this error:

2004-04-28 21:53:09 1BIQbt-0000qF-00 == help@??? T=remote_smtp defer
(0): SMTP error from remote mailer after RCPT TO:<help@???>: host
oxmail.ox.ac.uk [163.1.2.161]: 451-Invalid HELO (**my-server-name**) used by **my-server-ip**. See
RFC2821 S4.1.1.1 for syntax\n451-of the HELO command. This is a temporary error. You may try again
after\n451 this server (**my-server-ip**) has been fixed.

I have spoken to computing-services at Oxford at they say it's a fault at my end. This is strange as
I haven't changed my configuration or upgraded exim for months.

Does anyone have any suggestions, is it a fault at my end at all?

Thanks,

Tom Playford