To solve the security problems of IEEE802.11 wireless LANS, a email monitoring system is designed based on Microsoft Visual C++ 6.0. The system consists of three parts:email message listening, email message disposing, and email message reverting. This paper mainly studies how to deal with the captured email messages.A triplex filtrating method is proposed to ensure the integrality and veracity of the email original information, which includes protocol filtrating, port and sequence number filtrating, and time control rule. The system is well validated by test results.