BULK E-mail ??

I have a user that keeps getting his e-mail that sent to him mark as bulk?? Does our exchange server mark it as Bulk or does the sender mail server do that ? Also we have a barrecuda spam ware firewall before the exchange could it be that ?? The e-mail that is being sent to the user is address to them and them only and not Bulk mail but it has [BULK] in the subject line how can I fix this??