Email appearing on another computer

This is a strange email issue which I can't explain.

My client has Windows 7 they use Outlook 2007  on pop email.

They have been sending emails all day as per normal every thing seemed fine.
On the same network connected via WiFi is a Macbook. It has an exchange email account that is on a totally different email domain.

Three of the emails which my client sent from Outlook 2007 over the course of the day have somehow magically appeared on the Macbook. All three emails which the Macbook received were sent to different email address

I have never had an issue like this before.

Any ideas?
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Inspecting the headers of one of the received mails will give you an idea of what happened.
Mohammed KhawajaManager - Infrastructure:  Information TechnologyCommented:
Addresses could have been BCC'd but take a look at the message headers.

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AlwayslearningmoreAuthor Commented:
The address was BCC's thanks so much I feel like a royal idiot not picking this up.

Thanks guys
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