How to detect invisible users in Gmail?

Are gmail messages encrypted within Google's network (between Gmail users)?

• In other words, if I send an email from my Google account (gmail or Google Apps domain) to someone also with a Google account, does the email stay encrypted? My assumption is yes (why not?), but I'm curious why Google doesn't advertise this fact more. It is a huge advantage over Exchange for example (where communication is only encrypted between users on the same server/domain). I know the communication between me and the server is encrypted, I'm specifically asking about the message as it travels between Google users (I know it doesn't stay encrypted once it leaves Google's servers and goes to http://yahoo.com or whatever).

• Answer:

  It's more correct to think of Gmail as unencrypted all the time. The fact that connections to Gmail are encrypted by SSL is entirely passive. It has no impact whatsoever on the message itself which is not at any time encrypted and Gmail does not provide any encryption options I'm aware of. The message on Google's servers is either effectively or actually completely in the clear. At one time around 2004, Google added some semi-hidden features for digital signatures that fairly quickly disappeared, but no encryption functionality has ever been added to Gmail in public anwyay.