How is ZixCorp able to provide email encryption without a password?
Two minutes of your time on Google (no fancy search terms, just "zix gmail") would have told you that Google have been doing some work with Zix to bring some features to their Gmail product... thus you can assume they have Zix integrated. ;-)
Reading the product description on the Zix website, you can inferr that both sender and receivers need Zix servers if the messages are to be seamlessly encrypted/decrypted... otherwise the recipient will need to take further steps.
It seems you just happened to be lucky you were using an email provider that has Zix integrated so that you could see the seamless decryption as Zix intended !