Review for Limit non-BCC recipients by Borgers
Rated 5 out of 5 stars
Works exactly how you would like it... it prevents sending e-mails to a lot of people using TO or CC instead of BCC.
It prevents making a stupid mistake when your used to sending a lot of mailings with Thunderbird.
This is a great app for this single goal.
To create your own collections, you must have a Mozilla Add-ons account.