For starters, they don't keep an IP log, so there is no way for anyone to know what email is actually linked to you. The email is encrypted from their server to your PC, so it can't be read on the way to you. You can also easily send encrypted messages out to people on other networks.
We support sending encrypted communication to non-ProtonMail users via symmetric encryption. When you send an encrypted message to a non-ProtonMail user, they receive a link which loads the encrypted message onto their browser, which they can decrypt using a passphrase that you have shared with them. You can also send unencrypted messages to Gmail, Yahoo, Outlook and others, just like regular email.
The best would be to have both users in the same encrypted system, but it isn't needed.