It's looking like the classic untrusted domain stuff again.
If you're sending email directly from the server then hotmail and google need to know it's genuine. This is set up nowadays via an SPF record in DNS.
Yahoo are more lenient in their domain checking whereas hotmail and google are right on it.
Chris's email was bound to come through as its on the same domain.
There's definitely an email delivery problem and so far by asking those affected it does seem to be Hotmail and Google accounts (either directly or indirectly, ntlworld and my own domain are Google Apps based sites and still use the Google mail infrastructure).
Just using the tools at mxtoolbox.com, the site has no SPF records which is almost certainly causing this issue.
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