If there's one concept traditional direct marketers have had trouble grasping, it's e-mail reputation and how it affects deliverability.

The reason? It requires thinking that's completely alien to them.

Imagine if the U.S. Postal Service decided it would deliver commercial mail based on the senders' list hygiene and people's reactions to their campaigns.

That's what's happening in e-mail.

As a result, old-school DMers often stumble when it comes to permission practices. Most have embraced permission-based e-mail marketing — albeit some by force. But many think once they've received someone's OK to send e-mail, they've got the green light for all time.

Not true. Here's why: The number of recipients who hit the “this is spam” button is the top metric e-mail inbox providers such as AOL, Yahoo!, Google and Microsoft use to determine if incoming e-mail is solicited. By all accounts, a spam complaint rate of 0.5% or higher will cause serious delivery troubles. So even if the mailer has permission, if recipients find a campaign annoying or irrelevant, chances they'll report it as spam rise.

The good news, experts say, is that e-mail reputation is completely within a marketer's control. It's often compared to credit scoring. The higher a marketer's score, the better its campaigns will be treated by Internet service providers.

Besides spam complaints, what makes up a marketer's reputation score?

“Every ISP has its own algorithm of what it's tracking,” says Rick Buck, director of privacy and ISP relations for e-mail service provider e-Dialog.

One big factor is a marketer's unknown-user rate, which indicates how much mail is sent to addresses that no longer exist. E-mail campaigns with high unknown-user rates will have problems with deliverability.

Buck says ISPs want to see a 90% delivery rate.

“If you start bouncing at 20% or 30%, all of a sudden your list is taking on the characteristics of a scraped list,” he notes — a reference to the common spammer tactic of “scraping” or harvesting e-mail addresses off the Internet. “That sets off all kinds of alarms at ISPs.”

Here's where many marketers balk. Maintaining a high deliverability rate means cleaning inactive e-mail names from the file. “We're still getting pushback when we tell people they have to dump certain names,” Buck says.

But he's quick to add that mailing to inactive names is too risky. “If you continue to mail to someone who hasn't opened a message in six months, it may have turned into an expired e-mail address. So now it's a bounce. Worse yet, it may have been eighteen months since they opened, and now the address has turned into a spam trap.”

Internet service providers turn abandoned e-mail addresses into spam traps. If a marketer hits enough of them, difficulties with delivery are sure to follow.

However, even if inactive addresses haven't been turned into spam traps, mailing to them increases the likelihood of generating high spam-complaint rates. So when should an address be declared inactive and removed? “You need to figure out the pattern of people who respond and interact with you,” Buck says. Once that pattern is set, spotting inactives gets easier.

Another factor that affects reputation is how long the sending IP address has been in existence. Spammers typically set up new IP addresses, send millions of e-mails from them, and then move on. Because of this, ISPs are wary of new IP addresses and will “throttle,” or delay, the acceptance of e-mail from them until it can be established that they aren't spamming.

An e-mailer's reputation can also take a hit if its messages aren't authenticated. “Fifty percent, 60%, maybe 70% of leading marketers now outsource e-mail to an ESP and all ESPs authenticate,” Buck says. “The other 30 or so percent of legitimate marketers have gone to enough seminars and read enough articles saying they have to do it, that they've done it by now.”