This error occurs when the recipient email in the "TO" field is present in the "BCC" field. Certain email providers do not allow the email to contain duplicate recipients.
Additionally, this error may occur if the email address in the "BCC" field does not exist.
To resolve the error, ensure that the recipient's email address is not the same as the "BCC" email address.
If the email addresses in the "BCC" are unique, verify the validity of the email addresses specified in the "BCC" list.
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