This is what an signed message looks like, if you know and trust (i.e. have verified and signed this key through GnuPG command line or a graphical front-end like kgpg or WinPT) the signer's identity. We can see the mail is signed looking a the yellow pen symbol on the right (a right click on it will tell you even more). If you don't know yet a signing key, Enigmail will offer to look it up in one of its configured keyservers, but you will of course have to be sure (and to concretize this trust signing it with your private key) the public key you got corresponds to the owner it says, before you can trust its signatures!