Return Path will take over the front-end sales and marketing of Bonded Sender, an email white-list backed by major email providers including Microsoft and AOL, while IronPort continues to take care of the back-end technical systems.

Under Bonded Sender, legitimate companies that want to send legitimate marketing email to legitimate subscribers hand over a cash bond, which IronPort reserves the right to keep if the sender breaks its anti-spam policies.

In return, the sender gets a backstage pass for some of the world’s largest email services. ISPs that subscribe to the service agree to let bonded senders’ emails through their spam filters, on the basis that the user most likely requested the email.

IronPort vice president of marketing Tom Gillis said that the receipt-side users are ok with Return Path, a provider of email marketing consulting, taking over the customer-facing side of Bonded Sender. That support was a prerequisite of the deal.

We needed the large ISPs to back us up here, because we’re basically punching holes in their defenses, Gillis said. We have a lot of faith in Return Path.

The are now more than 250 million email accounts using Bonded Sender, and the number of send-side customers, has doubled since Microsoft Hotmail threw support behind the system last May, Gillis said. AOL is now also a receive-side customer.

Yahoo! Inc has also expressed moral support for the system, even though it is not yet a customer, Gillis said. Google Inc is said to be closer to becoming a customer, though there has been no official announcement of that.

But the Bonded Sender customer base stands at only around 300, partly because IronPort says it is laser focused on selling its core anti-spam appliances to IT and security departments, rather than selling email marketing tools to marketing departments.

So far, only three bonded senders have been ejected from the program, and only about $1,000 of bonds have been debited, Gillis said. This is due to the rigorous criteria to become a bonded sender in the first place, he said.

We’ve been pretty clear this is for only the best of the best, Gillis said. Over time we may relax some standards, or open it up, but the primary focus for us is trust.