I want both the emails and it’s replies to be encrypted.

Security of the data sent these days holds o guarantee to be entirely secured. So, it is a very important and intelligent step to be using encryption as a protection media. For this, you can use Outlook express or any such email client that gives you assistance in S/MIME which provides you with encryption only if you are capable enough to obtain or posses a digital ID which can also be referred as a digital certificate. There are definitely complex procedures for this. But it is almost insensible to spend so much of time, effort and above all money on it. Imagine buying a lock more expensive than the things inside them. It will sound something similar. There are simpler alternate methods of encryption with just a click of button on Outlook.