This is to bring up an important issue of privacy. Which are going to get much more important after February 2018 when the new privacy laws come into effect.
A small competitor of ours, have done the right thing and has sent out an email where they admitted having made a serious error by releasing in a bulk email the addresses of all recipients who appeared in the "to" section of the email. See the yellow arrow
What they should have done is put these emails in the Bcc line marked in green. What they have done does not follow the correct protocols required by the Australian government as they have broadcasted peoples email accounts.
I suggest that from this day forward, you be very careful, and if you are sending emails in bulk, you consider using Bcc rather than To or Cc. If it does happen make an investigation into the circumstances, try to determine how this happened and review your current processes and procedures to ensure suitable handling of personal information in the future as unfortunaly I cannot see any technological solution at present, there is no way I know of setting limits to the number of people in the To and Cc.. lines.
Remember legally you are reponsible for the privacy of the information you hold.
Technical note: This will outline the problems with Gmail making a technical solutiuon which I assume will be similar to others.
If you are using GMail you can have up to 2000 recipients in an email. There is a further limit that only 500 of them can have an email address that is not @gmail.com. If you are using group email addresses which can contain thousands of emails, this will be counted as ONE recipient, even though the email is received by many people. I cannot see anywhere these limits can be changed in Gmail.