Dear Email Client Developers,
If you allow your users to send an email with more than 10 email addresses visible in the TO or CC lines without throwing up a warning that says:
Hey, this is a bad idea. People will likely be upset that you just sent their email address to all the people that are getting this email. Please consider using the BCC field (or creating a mailing list at MailChimp)
You are not being a responsible email client developer.