Typically, this is the result of an invalid character within one of the email addresses. To resolve, review the email addresses and remove any invalid characters. You may refer to this WikiPedia article on which characters are considered valid and invalid. Note: We provide links to third-party websites in an effort to help you resolve your issue. We are not responsible for the information on third-party websites and we cannot assist with implementing resolutions from these websites.

Alternatively, you can export the email list to Excel and utilize an XML validator tool to quickly determine which characters are prompting this error.