Email encryption advice

Email encryption advice required. I use [login to view URL] as our hosting provider. I have various domain names hosted with them. I am in the process of trying to do business with certain types of organisations, however they state my email communications must be encrypted. I have various email accounts. I am looking for someone to provide me with advice on how to setup email encryption and which is the best email client program to use. I have previously used Newton Mail as our email program, which i find very easy to use and setup, however as i understand it does not support email encryption standards.