Uses maps to do a matching comparison on big datasets

maps.ex

contacts = Enum.group_by(contacts, fn c -> c["email"] end)

for lead <- leads do
  matching_contacts = contacts[lead["email"]]

  if matching_contacts do
    # Return a tuple with the lead and any contacts that match
    {lead, matching_contacts}
  end
end
|> Enum.filter(& &1)