Just looking through the manual I saw the following regarding Auto-Delete and passive probing: --------------- Passive address probing is very similar to active probing, but it is not tied to subscription renewal. Passive probing is enabled by default for small lists (e.g., <1K subscribers) but not for large ones due to the fact that passive probing does cost additional resources and large lists are often used for one-shot mailings where it is simply not effective to use those resources to probe addresses that will not be used a second time. --------------- We use passive probing for our lists (some of which have nearly 12,000 subscribers) and plan on using it for much larger lists soon (over 150k). Is this a bad idea? Is it a big resource hog? Thanks, Adam