Interesting Microsoft Outlook Warning

Hi to all,
when my boss starts microsoft outlook for just first time sending a message, it gives a warning , just happens for a first time
could you help me I couldnt understand why, it sends and receive messages but the warning occurs only for my boss s computer not another computers, so I must fix it
the warning as belove:

Some the files in the Web page aren't in the expected location. Do you want to download them anyway? If you're sure Web page is from a trusted sorce,click Yes.

when I click Yes , it shows the message again, then click Yes one more, it starts normally, and working normally...
have you any idea pls reply
thank you