E-mail, in particular SMTP (Simple Mail Transfer Protocol) has become an integral part of our lives, people routinely rely on it to send files, and messages. At the inception of SMTP the Internet was only accessible to a relatively small, close nit community; and as a result the architects of SMTP did not envision problems such as SPAM and sender-spoofing. Today, as the Internet has become more accessible, scrupulous people are making use of flaws in SMTP for their profit at the expense of the average Internet user.
There have been several attempts to bring this ancient protocol in-line with the current society but the problem of spam keeps creeping in. At first people had implemented simple filters to get rid of SPAM but as the sheer volume of SPAM increased mere filtering became impractical, and so we saw the advent of adaptive SPAM filters which automatically learned to identify and differentiate legitimate email from SPAM. Soon enough the spammers caught on and started embedding their ads into images where they could not be easily parsed by spam filters. AOL (America On Line) flirted with other ideas to control spam, imposing email tax on all email which would be delivered to its user. It seems like such a system might work but it stands in the way of the open principles which have been so important to the flourishing of the internet.
There are two apparent problems at the root of the SMTP protocol which allow for easy manipulation: lack of authentication and sender validation, and lack of user interaction. It would not be difficult to design a more flexible protocol which would allow for us to enjoy the functionality that we are familiar with all the while address some, if not all of the problems within SMTP.
To allow for greater flexibility in the protocol, it would first be broken from a server-server model into a client-server model. That is, traditionally when one would send mail, it would be sent to a local SMTP server which would then relay the message onto the next server until the email reached its destination. This approach allowed for email caching and delayed-send (when a (receiving) mail server was off-line for hours (or even days) on end, messages could still trickle through as the sending server would try to periodically resend the messages.) Todays mail servers have very high up times and many are redundant so caching email for delayed delivery is not very important. Instead, having direct communication between the sender-client and the receiver-server has many advantages: opens up the possibility for CAPTCHA systems, makes the send-portion of the protocol easier to upgrade, and allows for new functionality in the protocol.
Spam is driven by profit, the spammers make use of the fact that it is cheap to send email. Even the smallest returns on spam amount to good money. By making it more expensive to send spam, it would be phased out as the returns become negative. Charging money like AOL tried, would work; but it is not a good approach, not only does it not allow for senders anonymity but also it rewards mail-administrators for doing a bad job (the more spam we deliver the more money we make). Another approach is to make the sender interact with the recipient mail server by some kind of challenge authentication which is hard to compute for a machine but easy for a human, a Turing test. For example the recipient can ask the senders client to verify what is written on an obfuscated image (CAPTCHA) or what is being said on a audio clip, or both so as to minimize the effect on people with handicaps. It would be essential to also white list senders so that they do not have to preform a user-interactive challenge to send the email, such that mail from legitimate automated mass senders would get through (and for that current implementation of sieve scripts could be used).
In this system, if users were to make wide use of filters, we would soon see a problem. If nearly everyone has a white list entry for Bank Of America what is to prevent a spammer to try to impersonate that bank? And so this brings us to the next point, authentication, how do you know that the email actually did, originate from the sender. This is one of the largest problems with SMTP as it is so easy to fake ones outgoing email address. The white list has to rely on a verifiable and consistent flag in the email. A sample implementation of such a control could work similar to the current hack to the email system, SPF, in which a special entry is made in the DNS entry which says where the mail can originate from. While this approach is quite effective in a sever-server architecture it would not work in a client- server architecture. Part of the protocol could require the sending client to send a cryptographic-hash of the email to his own receiving mail server, so that the receiving party's mail server could verify the authenticity of the source of the email. In essence this creates a 3 way handshake between the senders client, the senders (receiving) mail server and the receiver's mail server. At first it might seem that this process uses up more bandwidth and increases the delay of sending mail but one has to remember that in usual configuration of sending email using IMAP or POP for mail storage one undergoes a similar process, first email is sent for storage (over IMAP or POP) to the senders mail server and then it is sent over SMTP to the senders email for redirection to the receivers mail server. It is even feasible to implement hooks in the IMAP and POP stacks to talk to the mail sending daemon directly eliminating an additional socket connection by the client.
For legitimate mass mail this process would not encumber the sending procedure as for this case the sending server would be located on the same machine as the senders receiving mail server (which would store the hash for authentication), and they could even be streamlined into one monolithic process.
Some might argue that phasing out SMTP is a extremely radical idea, it has been an essential part of the internet for 25 years. But then, when is the right time to phase out this archaic and obsolete protocol, or do we commit to use it for the foreseeable future. Then longer we wait to try to phase it out the longer it will take to adopt something new. This protocol should be designed with a way to coexist with SMTP to get over the adoption curve, id est, make it possible for client to check for recipients functionality, if it can accept email by this new protocol then send it by it rather than SMTP.
The implementation of such a protocol would take very little time, the biggest problem would be with adoption. The best approach for this problem is to entice several large mail providers (such as Gmail or Yahoo) to switch over. Since these providers handle a large fraction of all mail the smaller guys (like myself) would have to follow suit. There is even an incentive for mail providers to re-implement mail protocol, it would save them many CPU-cycles since Bayesian-spam-filters would no longer be that important.
By creating this new protocol we would dramatically improve an end users experience online, as there would be fewer annoyances to deal with. Hopefully alleviation of these annoyances will bring faster adoption of the protocol.LILUG News WWTS 2008-03-16T22:12:08