Recently I deployed a campaign where 3585 leads were qualified in which 29 leads were blocked so the total sent should be 3556, but when I see reports I can see only 3479 as sent is there a way to find out why rest of them were not sent?
Grégoire is correct. Marketo will automatically avoid sending the email twice to the same email address to ensure you don't overwhelm your leads with the same content more than once.
In order to check which leads didn't receive it you can create a smartlist with the same criteria as previously and add filter such as 'not was sent this specific email' as well as is not unsubscribed, blacklisted etc etc'. You should be left with the lead that weren't sent the email but didn't get excluded because of being blocked.
It's also possible the data shifted and reduced the number who qualified, either through email fields or other reasons. The count is only valid at the time you ran it or at the time the batch executed.
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