sending group email; error: message cannot be sent because one or more recipients do not have email address

Original Title: sending group email

I have copied and pasted to my email "to"window a list of  email addresses from spread sheet. When I try to send, I get box that states that "message  cannot be sent because one  or more recipients do not have email address."Why can I not send the message to those who DO have address and  filter out the invalid  addresses?