Just looking through the manual I saw the following regarding
Auto-Delete and passive probing:

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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.
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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