Unlike content filtering methods such as Bayesian or lexical analysis, source filtering looks at the IP address of the email server that is trying to send an email. If that source is a known spambot or spammer’s MTA, the filter simply refuses the connection.

This way, the mail is never actually received. The new Connection Control, delivered as an update to the firm’s IronMail appliances, blocks about half of attempted connections in this way, according to vice president of marketing Mike Rothman.

We’re able to drop 50% of the connections with this, which basically doubles the throughput of our box from day one, Rothman said.

The feature is based on TrustedSource, CipherTrust’s source reputation database, which is compiled from the past behavior of IP addresses of servers that have sent email the company’s existing customers.

In the past, TrustedSource could be used as one data point used in scoring when IronMail was filtering already-received email. Much of the spam that gets past Connection Control is dealt with by the device’s existing message-level filters.

Similar offerings are already on the market from appliance vendors such as IronPort, which offers connection-level throttling, and MiraPoint, which offers a connection-level variation on the challenge-response concept.

One of the closest comparable offerings to Connection Control, functionality-wise, appears to be Postini Inc’s managed email security service. The two companies compete in slightly different markets, however.

CipherTrust’s Rothman said: What’s different about what Connection Control is doing is that it is giving the user control over what they consider is spam. It enables users to make local source-based decisions based on the type of spam, he said.

CipherTrust has also announced that its sales were up 17% in the third quarter. The announcement follows claims made by rival IronPort last week that its own sales had outstripped CipherTrust’s in the same period.

As Rothman points out, without public company oversight, the claims of private firms mean little. Neither firm discloses its revenue. IronPort had claimed it booked $16 million and that CipherTrust had booked $14 million, which Rothman disputed.

Rothman would not confirm or deny rumors that CipherTrust is currently preparing its IPO papers, but said: A company that’s been profitable for nine quarters in a row has a lot of options, and trading on the public markets is one of them.