Email is sent via a protocol named SMTP, which stands for Simple Mail Transfer Protocol. Unfortunately, because of its simple nature, it is prone to the kind of abuse that pollutes our mailboxes every day. One significant abuse vector that has been gaining attention due to phishing attacks is the practice of authenticating the sender of an email message.

When authentication was being developed, there was a number of technologies proposed. After some consolidation, there are now two leading methods to authenticate your emails:

Set up your SenderID
Developed by Microsoft and a company called Pobox, SenderID takes moments to set up and is a simple way to authenticate your messages via text entries in the domain name system.

Microsoft is driving adoption of SenderID by implementing authentication checks in both MSN and Hotmail. Messages that pass authentication checks are given a slight "bump" to get delivered to the inbox. Messages that fail authentication checks are branded with a warning to the end user. 

SenderID notice in Hotmail

Use DomainKeys
DomainKeys were developed by Yahoo! and Cisco Systems as a method to cryptographically ensure that email messages are authenticated and have not been altered. While DomainKeys require more effort to implement, the digital signature is seen as the preferred method of authentication and is quickly gaining wide adoption.

Similar to Microsoft and SenderID, Yahoo leverages its considerable mailbox universe to encourage the adoption of DomainKeys. By tying it into its internal anti-spam system, Yahoo gives messages with DomainKeys a better chance to make it into the inbox and confirms the authenticated status of the message to the reader.

Yahoo's feedback loop, a system of providing user complaints back to the sender, will require DomainKey presence when it goes live.

DomainKeys notice in Yahoo

Increase your reputation score
 
IP-based reputation systems have become popular for fighting spam at ISPs. Think of reputation systems as a credit score for your IP. In this case, instead of using the score to qualify you for a loan, the ISP is looking at your score to see if they should accept your email.

Reputation systems are unique and internal to each ISP that uses them, which includes the top players: Yahoo, AOL, MSN, Gmail, et cetera. With the exception of Microsoft, ISPs guard their reputation systems, so unless you are blocked outright, it's not easy to determine how a specific ISP sees your reputation. A low reputation score may cause your messages to be throttled, filtered or blocked outright.

Microsoft SNDS reputation data. Green results are excellent, green/yellow is average, and yellow/red results indicate poor reputation.

One of the biggest factors that affect an IP's reputation is the spam complaint rate. Nearly every ISP and webmail provider has a "report spam" button built into its user interface. When readers hit those spam buttons, the ISP counts that as a spam complaint and keeps track of how many complaints are generated from messages coming out of each IP. If complaints exceed a certain threshold, which can sometimes be as low as 0.01 percent, the reputation score for that IP is lowered.

Keep your score high
It's crucial to set up feedback loops with all the ISPs that offer them (more on this in a bit) so that you are aware of how readers are reacting to your messages.

The second major factor of IP reputation is the ratio of invalid address attempts to the number of valid mailboxes reached. An IP attempting to send mail to non-existent addresses and generating tons of bounces looks very much like a dictionary attack (when a hacker systematically tests all possible passwords) and has a very high chance of running into temporary or permanent blocks.

To combat bounce spikes, pay attention to list hygiene. Annual list turnover has been quoted at as much as 30 percent, which means that after a year, a third of your email addresses will be invalid. Regular mailings, at least monthly, help prevent bounce spikes that may occur if a mailing list is used infrequently.

Correctly interpreting the bounces you receive is paramount to ensuring that you're not mailing to deadwood. Bounces can be categorized as soft, indicating a temporary problem (mailbox full), or hard, indicating a permanent failure (invalid user). Make sure that your bounce processing system can distinguish the two and remove the hard bounces from your mailing list.