Can't send email to specific email address

Hi,

I have a problem with a specific email address. I am unable to send an email
to it (I am however able to recieve emails from it). When I send an email to
this address, it appears in my "sent items" folder and is not bounced back to
me, but the recipient does not receive it. This has started fairly recently;
in the past I was able to send this address an email. I am using Outlook 2007
and Vista. If I send an email to this address from my iphone, they receive
it. The iphone has the same incoming and outgoing server settings as Outlook
on my PC.

Any help would be greatly appreciated!
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Cruise Planners