Rated 5 out of 5 stars

This is one of the tools that are so useful and simple that you wonder why the feature is not included in the Thunderbild core itself. Sending out an email to loads of people with the recipients' addresses visible to everyone is not just very annoying, it can be a major breach of privacy protection. With this add-on installed, it just can't accidentally happen anymore.

The settings enable you to define your own level of restriction: You can set the maximum number of recipients in the "To" and "CC" fields that may pass without warning (10 by default), and you can choose wether you want to be just warned (default) or stopped when that limit is exceeded.

There's only one item left on my wishlist:
For some reason that is a mystery to me, Thunderbird still doesn't have a built-in mail redirect (= bounce) function, so an add-on, usually Mail Redirect, is required to fulfil this basic task. When one bounces mails using Mail Redirect, the Use Bcc Instead add-on doesn't work. I have no idea wether it would be complicated to make the two add-ons work together, but that would make it perfect.

This review is for a previous version of the add-on (1.2). 

For your review. I wrote this to"fix" a problem a client was having with an employee who was not paying attention. I am glad others find it of value.

I am not familiar with the Mail Redirect addon or how it works or why this addon doesn't play nicely with it. But I know that this addon doesn't work properly with the excellent Send Later 3 addon. I have communicated with the author of that addon and perhaps we can find a way to resolve this. However, there are technical difficulties that may prevent addons which work by intercepting mail send event from ever getting along perfectly. I guess there are always limitations.