Email is unsafe, it's a harsh realization that took way too long to hit the mainstream. And I'm not sure most people fully understand that email is just plain text and that it is sent over Wi-Fi networks as plain text, which means that anybody who can snoop (by simply being connected to the same network while you send or receive) your network can litterally read your emails.
If you've ever sent a password or sensitive information to your co-workers or to your family via email, you should assume that data could have been accessed. It might be chilling, but it's basically as secure as snail mail, which you left in a mailbox for hours before it was picked up by a post office worker whose trustworthiness you took for granted. Your letters were then transfered to a distribution center with varied levels of security and surveillance, within which prying eyes could very well have looked at your private communications.
GPGTools allows you to seamlessly encrypt and decrypt email messages within Apple's Mail.app with people you trust by using a private & public key pair just like Git does. It will encrypt your messages with your trusted contact's public key so that only they can decrypt them using their private key. Likewise, the other way around. Your contacts will be able to send you encrypted messages using your public key.
You can publicly share your public key (as I do) as long as you can trust that someone won't swap it for a malicious key, so make sure you trust the medium — your own site if it's secure or publication platform that offers two-factor authentication.
Make sure to read GPGTools' Quickstart Guide after you install the suite. They're planning on improving the GPG Keychain Access app in the near future and revamp this guide but it remains the best place to get started for now.
I know that I recommend Airmail as a communication tool, but while Mail.app can be more secure when used in conjunction with GPGTools since Airmail doesn't implement PGP & s/mime yet (they will soon). Mail.app's user interface is leagues behind that of Airmail. As long as you have to sensitive information to discuss or credentials to share, I would recommend sticking with Airmail for most email jobs.