Looking for 'People' who have these types of details. 
Question
How to Filter Contacts with "Blank" email. 'Unknown' does not bring up the Leads with no email present, and searching for a blank field is not something that is coming as obvious for me.
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I tested this to double-check for you! If you aren't getting results, I would suggest checking that those leads you have in Contacts actually don't have emails attached to them. You can check this by adding 'Emails' into the columns in Contacts here 👇🏼
If there are leads with no emails showing there - I would suggest reaching out to one of our support agents via the Messenger with a few of these examples so they can investigate this for you 👀