Hi Lyndal_Bland
Thank you for reaching out. We appreciate your patience and understanding on this matter. The possible caused of the error you are receiving could be the email address was rejected by the server or the email address is spelt incorrectly. It's probably a good idea to take addresses that throw this response off of your main list, as it's likely a bogus address or one that was mistyped. Note to clear cache first before sending emails.
Please feel free to post again I am happy to assist.
If my response has answered your enquiry please click "Accept as Solution" to assist other users find this information.
Cheers,
Princess