msg['To'] должна быть строкой:
msg['To']
msg['To'] = "a@b.com, b@b.com, c@b.com"
Хотя recipients в sendmail(sender, recipients, message) должен быть списком:
recipients
sendmail(sender, recipients, message)
sendmail("a@a.com", ["a@b.com", "b@b.com", "c@b.com"], "Howdy")