[external] tag is not appearing for one user.

Hi,

one of our user seems to have a problem with [external] tag. Thing is when customer mail him and his colleagues with totally new topic he receive email without [external] tag. This situation cause problem because then if his colleague answers this mail, which for him e-mail is properly taged as external, it open new conversation as topic of this e-mail is RE: [external]xyz... What can be root of this problem? It seems to me as Outlook client related but I can't find solution.

Will appreciate every help.

Best regards.

Adam