There is nothing in YPN's TOS that forbids the use of popunders from another network as long as they are not excessive.
The Web page for implementation must meet the following guidelines: The Ad Unit: The Ad Unit cannot be obscured in any way The Ad Unit must be shown in the requested page, not as a pop-up or pop-under window or in an e-mail The user must be able to tell the difference between the Ad Unit and other content on the publisher's page. The Ad Unit must be clearly labeled as "Ads by Yahoo!" and this label should not be confused with other advertising or links on the publisher's page. Images or other media cannot be associated with the Ad Unit. The Ad Page: The Ad Page must be content based, fully functional and not contain any broken links. Non-content pages include domain registration pages, welcome pages, error pages, and Web content generated in or through software applications. The Ad Page must be in English or Spanish and be consistent with the language of the Listings. The Ad Page must respond adequately to user requests. The Ad Page must not actively encourage users to click on Ad Units. The Ad Page must not disable the browser's back button. The Ad Page must not spawn more than one partial pop-up/pop-under window and this window must not interfere with user navigation. The Ad Page must not automatically replace the browser homepage. --------------- That should answer any question you might have. - Compu