No idea what caused mass change Subscription Newsletter = True to False

[44MattWolf44]

Hello all

Question here. Last month we had about 98k Newsletter subscribers (criteria Subscription Newsletter = True). Now we have 75k, and I cannot for the life of me figure out why. 

I use the filter below on a Smart List to try to see everyone this impacted, but it's telling me this is applicable to only 677 people

I should mention that we opened up our recipient pool for the newsletter quite a bit recently and had a total of 8k unsuccessful deliveries. I realize that's a lot, but if those unsuccessful deliveries impacted their subscription status, wouldn't it be reflected in this Smart List? Also, even if the 8k list has duplicates, would it seriously account for 23k leads? 

Thanks in advance for any and all help

[Darshil_Shah1]

Could by any chance these people got deleted off from your Marketo's DB? Also, unsuccessful deliveries can affect person’s Email Invalid field, that should not automatically change the custom Subscription Newsletter field.

[Katja_Keesom]

Did you by any chance send a newsletter to that 98k audience? In that case you can filter your smart list on people who were sent that email and are no longer part of your subscriber base. That might help narrow down what happened to them in the activity log.