There are well over a hundred spam-blocking tools available, and the number seems to grow each week. Some of the tools are more interesting, effective and useful than others. Here\u2019s one of them: Secluda\u2019s InboxMaster.InboxMaster is SMTP gateway software that works with a variety of operating systems - including Windows, Linux, Solaris and Mac OS X - and it requires no software to be installed on client machines. However, as is the trend in newer spam-blocking tools, InboxMaster allows individual users to completely control what is considered spam or what is considered good e-mail.After processing a message, InboxMaster can relay it to the recipients\u2019 mail servers or other pre-specified servers on different ports, effectively isolating a mail server and preventing it from receiving e-mail from any source other than InboxMaster. The ability to relay e-mail to servers on different ports adds a level of security to the mail delivery process, acting something like a firewall for mail delivery.InboxMaster allows individual users to classify incoming messages as from a trusted source, from an unknown source, or from a source that should be blocked.InboxMaster also has a \u201cstealth\u201d feature that allows a user to automatically generate a customizable 550 error when a user receives a message from a blocked source. This makes it appear as though the e-mail has bounced and that the user\u2019s e-mail address is bad, thereby (hopefully) reducing the effectiveness of directory-harvest attacks against a protected e-mail system. InboxMaster allows administrators and end users to trust or block messages based on a variety of attributes, including domains, individual IP addresses or ranges of addresses, wildcards and other attributes.One of the nice features of InboxMaster is its individual focus. For example, an administrator can block a domain, but can allow a single user to override that decision if he or she chooses.InboxMaster processes outgoing e-mail as well, learning from the behaviors of individual users to fine-tune its spam-blocking abilities. For example, let\u2019s say a message from firstname.lastname@example.org is blocked and his messages are placed into quarantine because Bob is not a trusted sender. Then, at a later date I send Bob a message. At that point, Bob will automatically be added to my trusted senders list, and all of his messages currently in quarantine will then automatically be delivered to me.InboxMaster includes a variety of reporting tools, and it allows administrators to manage the system via a Web interface. InboxMaster is priced at $25 to $40 per seat based on volume.Secluda is a relatively small company that many users may have never heard of before. However, the technology is definitely worth a look.