I'm digitally signing all my emails now, when you receive one from me it will contain the signature file with an extension p7s.
On a Mac this is stored in your keychain. If you were using Apple Mail then you would see a little tick in the top of the message.
If you were also signing your messages and had sent me your signature file then we could also send encrypted messages to each other.
It basically means if you receive an email from me your mail client will compare it to the signature on file and warn you if its not from me. It's all about making the Internet a more secure and pleasant place!
If you want to set up your own digital signature using Apples Mail app have a look at these instructions
http://www.joar.com/certificates/
Happy emailing!
ben@trilby.co.uk