Very strange problem in Mail App

[dockerslund]

Have encountered a really really strange problem with Mail App.
If I'm sending an email to xxxxx@kineticeducation.com.au, then it's not received, but only if it's sent from either xxxxx@zealandmedia.com or xxxxx@thebrandbox.biz (they're both Google Apps mails by the way - the kineticeducation one isn't).
Any other email adresses can easily send to the kineticeducation email, when sent from Mail App.
Now here's the strange thing. If I log in to Gmail.com (as either zealandmedia or thebrandbox), then I can easily send to the kineticeducation email.
I've also set up those two email addresses in Mozilla Thunderbird and Windows Live Mail. They're received on the kineticeducation email with no problems.
So it's only when I send FROM zealandmedia.com or thebrandbox.biz TO kineticeducation from WITHIN Mail App.
Please heeeeeeelp

P.S. No bounce back errors are received, when I send from Mail App - and just to confirm that I can send to any other email address from zealandmedia.com or thebrandbox.biz within Mail App - just not kineticeducation

P.P.S. Obviously the email addresses aren't 'xxxxx'