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.
If you are only getting the error with specific contacts, we’d be happy to look into this further with you.
Please feel free to email us at email us at webservices@constantcontact.com, referencing case number 27616858, and include the account's username, and any example contacts whose email addresses are being affected. If you are also able to send us the full payload, that would be best. We look forward to assisting you!
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