Sometimes “permission slips” aren’t enough when it comes to e-mail deliverability.

In case you’ve been wondering how much marketing e-mail actually reaches its intended targets, a recently released benchmark report from e-mail scoring and certification services provider Return Path has some answers. It finds that only about 75% of “permissioned” e-mails are actually making their way through.

That means one in every four e-mails are either hitting a spam or junk folder, or are being blocked by ISP-level filtering.

The report was based on analysis of data from Return Path’s Mailbox Monitor service, which tracks the delivery, filtering and blocking rates for more than 600,000 e-mail campaigns.

Interestingly, the delivery stats for business-to-business marketing e-mail aren’t much lower than for business-to-consumer e-mail. This was considered somewhat surprising because of company-level filtering systems like Postini, MessageLabs and Symantec that are installed at many large corporations. Presumably, they do a more thorough job of filtering e-correspondence.

The Return Path report also included a few cautionary notes for marketers:

 Many e-mailers believe that whatever gets deployed and doesn’t bounce must be reaching inboxes. But senders are notified only when the e-mail is a hard bounce – not if it has ended up in a spam or junk folder.

 Relying on rented e-mail files in the B-to-B world can be dangerous, as those files can be riddled with spam traps. Commercial entities are always on the search for new prospects and leads … but merging a good in-house list with a few of these bad boy rental lists can result in compromising the entire database.

 In the consumer sector, many marketers aren’t paying close enough attention to inbox placement rates. For example, data about Gmail shows that while many marketers are ostensibly achieving a 90%+ deliverability rate, fewer than one in five of those emails are actually being directed to the “priority” inboxes within Gmail as designated by the recipients. And you can bet that precious few of the other ~80% are getting any sort of attention at all from consumers.

More details about the Return Path report can be found here – well-worth checking out.