ISPs are still feeling their way forming best practices around deliverability and sender reputation. A few constants, though, have emerged that e-mail marketers should heed. It’s better to trim a list to the more active readers than have your mail sent to the spam box. Always get opt-in permission and no cheating by merging email addresses given for a mail stream to a larger list.

These are a few of the insights in Exact Target's recent report [pdf] " Letters to the C Suite report: Getting Serious About Permission and Deliverability."

Bottom line: If a consumer did not directly provide you with his or her email address for a mailing list, do not send him or her email - no matter how reliable or lucrative your supplemental source of "opted-in" addresses may be, says Carlo Catajan, anti-abuse & delivery at Yahoo. One major reason why is that mailbox providers are starting to monitor open and click-through rates, using the data to augment the more conventional reputation data generated by bounces and spam complaints, he continues.

"This enables providers to better gauge which senders are delivering engaging content to their users that they want to receive. And that, in turn, brings them closer to the ultimate goal of delivering only wanted messages to their users' inbox." Eventually, senders who measure their mailings' success based on the quantity of subscribers on their lists, instead of the quality, will pay the infrastructural and incidentalcosts of that misguided notion, Catajan says.

If you still think that more recipients will help you hit your targets, consider this, says Geralmy Swint, senior network abuse engineer at Earthlink. "As much as 20% of your mailing list becomes inactive annually. If you don’t properly prune bounces, or keep trying to mail to old, outdated lists, the more likely it is that ISPs will notice you, and probably even block your mail."

While there are many ways providers are measuring reputation of a sender, they are all variations of a similar theme, George Bilbrey, co-founder and president of Return Path, says.

These include:

• The number of messages marked as spam relative to the number of messages in the inbox

• The number of messages in the junk folder marked as not spam (this is generally positive)

• The number of spam trap addresses that have been sent to by the mail server. Ideally, a spam trap is an email address that has never signed up for any messages from any source. Therefore, any messages hitting those spam traps are considered spam.

• The number of dead addresses ("unknown users") to which the mail server is sending.