The encryption, and signature-based sending. I have found that my emails are way more secure, and the encryption stops attackers from getting into my emails and reading the contents. The signature-based sending has also allowed me to prove to my clients that I am who I say I am by displaying a ribbon in the email header. This tells my clients it is the real me and it is safe to discuss what they need to discuss, all while encrypting communications. This is hands down the most valuable tool in my arsenal.