Message of error when I send message

Today when sending an e-mail to several people entered in the address book, an error message appeared when I sent it: 'The recipients entered in the ‘ccn’ field have an incorrect format and I’m thinking because the addresses are entered between apostrophes, which are not present in the address book. I must say, however, that the mail went out regularly. The question is why do I get this warning, as the dispatch starts regularly?