Yea, people mostly equate email to an electronic letter, but it's more like an electronic postcard. Anyone handling it can simply read it.
So you'll want encryption, too. So either you get everyone to use PGP/GPG or get them to use a privacy-by-default provider.
Good luck with the first option and I'm not sure how interoperable the various providers are, so in the worst case you'd have to rally everyone to the same provider.