Hello @user916600 ,
The "source" field is something our system automatically adds, depending on how the contact is added to your account- "Added by You" for manual imports, a named integration if it's one of our official ones, specific coded form type or landing page, other app (externally developed integration), etc.
Is the lack of "source" causing issues with how you manage or send to the contacts? Does exporting those source-less contacts (you just need the emails), then re-importing them force the system to update them as "Added by You?"
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