strange non delivered email problem

Hi,

I have a customer with a Windows 2007 SBS server who is having a problem with emails, what is happening is when an email is sent in to a user from a certain company it does not appear in hit Inbox yet if it is sent to anyone else in the company it does.

I have checked his rules and there is nothing I can see to suggest where the problem is.

All other emails work okay and he has a second mailbox inside outlook and this can recieve the emails from the company without any issues.

What am I missing?

Thanks