Mailing lists are used for information exchange in specific groups. However, in the recent times, the number of spam mails received has increased, and considerable amount of time is wasted in filtering spam mails. Spam filtering techniques are widerly used tool, however, they produce false positive and false negative results. We propose a system to block spam mails in a mailing list. In our system, we assign different posting addresses to different mailing list members. A mailing list member sends a mail to the mail address assigned to him for sending a mail to the mailing list. When a spam mail is received, the address that is the cause of the spam mail is identified and invalidated, and a new address is assigned to the member. Thus, we can block spam mails from the invalidated address. Furthermore, our system is highly compatibile with current mail systems because our system does not require any particular software to be installed in the client machines.