I just sent an admin mail to 6000 users :(

Jeroen,
Eeek, very sorry to hear you ran into issues after your migration. We agree that JFBConnect could look at that setting and use it, but it's not something we currently do, as you've noticed. There are lots of settings that could complement or conflict with JFBConnect features; we don't implement them all due to time or functionality requirements. This is simply one that we haven't had much user feedback that they don't want the emails sent on user registration.

As to the actual problem, it definitely sounds like a bad migration setting hit you. The sendEmail field's description in the admin area is "Receive System emails". The new user email is a system email, and we actually use the same code to send the email that Joomla in the com_users component which selects all users that have the sendEmail field set and simply spits out emails to all of them. com_users checks the mail_to_admin parameter first, but the code to actually send the emails is the same.

Additionally, many other extensions use the "Receive System emails" setting to determine how/where to send certain emails (comment moderation, article moderation, etc). It sucks that JFBConnect is the one that triggered the problem, but I think it would have happened sooner or later.. including if you ever temporarily switched the mail_to_admin parameter back on for one reason or another. Thankfully the admin emails don't have any security or sensitive information, like other system emails from some extensions can contain.

If you used a tool to perform the upgrade, I'd highly recommend contacting the developer to let them know they may be importing that setting incorrectly.

Thanks for the report though. I can see adding an option or check for the mail_to_admin parameter in a future version to let admins decide whether to receive the new user emails or not.

Alex