Too many addresses in an email group send - some failed to be sent, and now won't send at all?

Hi. I sent an email to a group of email addresses and the operations log told me there were too many in the group., so some failed to be sent. I have tried to forward the message to the remaining few which were “too many” - but now none of them will send, giving me an error message saying the server does not recognise their addresses? What Iam I doing wrong??

have you created a distribution list with the mail addresses? How many mail addresses?