Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
Constant Contact's email features were designed around 1) marketing to groups of contacts and 2) automation. As such, the only options for sending an email to an individual contact (via the API or the UI) would be to schedule an email to a list containing only that contact, or for that contact to trigger an automated email (such as by being added to a specific list).
Please let us know if you have any other questions!
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