Is there a way to turn off all e-mail notifications by default?

Since I added the new "match addon" feature, I'm concerned that people who don't realise they're following each other (because it's only happening on Facebook or Twitter, but they don't realise what it means in NationBuilder terms) will suddenly start receiving e-mails.

Would be great if they could all be switched off, and then individuals who want them can switch them on.

Thanks.