From a SMTP pov, the To and Cc header are not read. The real recipient is stated at the begining of a send mail transaction. It's up to the client to collect this recipient list. Sendmail does this by reading the To and Cc header. smtplib doesn't do this when being invoked with an explicit To array. This commit adds the Cc people to the recipient array. The issue has been introduced by the switch from sendmail to smtplib.