Chances are pretty good a consumer who hasn’t opened a marketer’s e-mail in three and a half years isn’t a viable prospect. Yet in a study of more than 100 retailers, almost one-third were still mailing to these individuals at exactly the same rate they had when their communications were being opened. And another 23% were sending out messages, albeit at a reduced frequency.

Fewer than half (46%) had stopped mailing entirely.

No harm done? Wrong and wrong. Internet service providers (ISPs) are increasingly giving weight to engagement metrics when determining deliverability. They know – and care – which messages are and aren’t being opened, even if marketers don’t. And messages that go unopened will increase the likelihood a marketer will be labeled a spammer.

Tolerances vary, but a mailing list that contains 50% inactives is at a fair risk for being filtered, according to the study, which was published by e-mail marketing firm Responsys. And marketers that regularly mail to larger files – those in excess of 1 million names – are at an even higher risk.

Having an opted-in list doesn’t necessarily provide a marketer with cover. Responsys estimates nearly one in five permission-based e-mails doesn’t make it into intended mailboxes, with the primary reasons for messages being diverted being poor list hygiene and shoddy management of inactive addresses.

In some cases it’s easy to understand a marketer’s confusion. Responsys recommends drawing distinctions between inactive subscribers and inactive customers. An e-mail subscriber who doesn’t read a marketer’s messages, but nonetheless goes on a Web site, or makes a purchase, will be tagged as an active customer. But the ISPs don’t know about these activities – and they don’t care. Their calculations are based entirely on how recipients interact with e-mail messages.

E-mail subscribers can take several actions that will lower a marketer’s engagement score, in the eyes of ISPs. If the message is reported as spam, obviously. But marketers are also penalized if the recipient deletes the message, moves it to a trash file, marks it as read or just ignores it.

More than anything else, though, ISPs look at clickthroughs, which are tangible proof of engagement.

Regardless of what a marketer might think, most ISPs consider e-mail addresses to be inactive after between 18 months and two years of inactivity, a time frame long enough to allow seasonal shoppers to receive messages during one or two sales cycles after sign-up. But this isn’t hard and fast: ISPs give a little more leeway to marketers that have higher overall engagement than those with lower scores.

Direct response marketers can use one of their great strengths – testing – to demonstrate the futility of mailing to inactive addresses. By segmenting their lists into their own definitions of active and inactive subscribers, they can track revenue, conversion, complaints and frequency with which addresses are turned into spam traps.

Over time, analyzing these results based on most recent activity on the part of the subscriber will provide insight into ISPs’ level of tolerance for their messages. Marketers shouldn’t take the above 18 months to two year figure as gospel: One retailer Responsys worked with found only addresses that reflected some sort of engagement within the previous nine months were low risk.

What can marketers do to boost their scores? Stop mailing to inactive segments is the obvious step. But they can also attempt a variety of re-engagement strategies. These range from presenting inactive members with the best possible offers to reducing the frequency of their messages.

High-value come-hither offers are an obvious tool, but reducing frequency is apparently less so. Consumers who may be overwhelmed with a daily flow of promotions may find a weekly digest consisting of highlight much more manageable. Over its 40-month study, Responsys found 55% of major retailers reduce the frequency of their mailings to inactive customers.

Marketers can also boost engagement asking recipients to “update their preferences”, such clicking through a link to enter or confirm their physical addresses or job titles, indicating their offerings preference (which is also a good way to expose recipient to new merchandise) or even giving them control over message frequency.

Marketers can also take the counter-intuitive step of elevating or highlighting their unsubscribe features, especially within messages going to sub-groups of inactive recipients. ISPs don’t count the number of unsubscribes against a marketer, as they understand testing the waters is part of the sales process. But they do count spam complaints and the like very heavily against marketers.

Re-engagement programs and messages that highlight the unsubscribe feature should be automatically triggered after a period of inactivity, and not done on an ad hoc basis when it occurs to a marketer – or, even worse, when the marketer is contesting being a spammer, according to Responsys.