If you do not have keyserver-options auto-key-retrieve set in your gpg.conf file and you read a message which is signed or encrypted, you will see a Pen icon in the headers display area with a Question mark on it, the Enigmail status line in the headers area will say Part of the message signed; click pen icon for details and the message in the Message Pane will show all the OpenPGP message block indicators and the signature block.
You may also see this if you have keyserver-options auto-key-retrieve set in your gpg.conf file and the OpenPGP key is not available on the default keyserver.
Clicking on the Pen and Question mark icon will bring up a window advising that the key is unavailable in your keyring. Clicking on OK will bring up another window with a list of keyservers from which you can select to download the sender's public key from.
To configure the list of keyservers you wish to use, go to Enigmail -> Preferences -> Basic tab and enter the keyserver addresses in the Keyserver(s): box, separated by a comma. The first keyserver in the list will be used as the default.
Further help is available on the Enigmail Help web page
If you have questions or comments about enigmail, please send a message to the Enigmail mailing list
Enigmail is open source and licensed under the Mozilla Public License