8.2.3. Retrieving the key that signed the message

A nice feature of Enigmail is that it can import automatically the public key needed to verify a message. If you received a message for which you don't have the sender's public key, as shown in the figure in Section 8.2.1, select from the OpenPGP status bar the menu command Details → Import Public Key, and Enigmail will offer to try to download from a keyserver the key that was used for signing:

Just click on Import and Enigmail will do that for you. The imported key will be added to your keyring.
More often, you will receive someone's public key as an ASC file attached to the email message. In this case, importing the public key is just as easy: you only have to right-click on the attachment and choose Import OpenPGP Key.
Someone might also send you his public key embedded in the message text.
If you want to do some signature (and encryption) tests, then you'll find a very patient correspondent in Adele, “The Friendly OpenPGP Email Robot”. Adele can be contacted at adele-en@gnupp.de and is an automated program that is able to receive and understand OpenPGP messages, and to reply to them accordingly in a very short time.
I sent a simple cleartext mail (unsigned, unencrypted) to Adele, and here's how she replied to it:

Here Adele complains that there was no public key attached to my message, so she doesn't know what to do with it. However, she provided me with her public key embedded in the message: note the OpenPGP block in the mail body, and the yellow OpenPGP status bar. Clicking on the Decrypt button, and then confirming, will import the public key into my public keyring:

Adele's public key is now in my public keyring.