Your email can be signed using your private key and verified by the receiver using the corresponding public key.
Successful verification proves that it was you who sent the message (as only you have access to your private key) and that the message hasn't been altered since it was posted and arrived in your friends browser.

You can sign a plain message, sign a plain message and then encrypt it and then sign the encrypted version!

Click the signing button as shown below:


You will then be presented with a list of your private keys (that you previously created)


Select a key by clicking the key Id and you will be asked for the passphrase for that key:


After you've entered the correct passphrase the message will be signed as below:


Send the message and the receiver if he has your public key (and the kodamail extension loaded in chrome) will see a green icon as shown below:


Clicking the green icon will verify the message using the public key:


If the message has been tampered with ie not the same as when signed, you will get a message like this:


You can test this yourself, right click on the message, then 'Inspect' will bring up the html source of the message.
Find the text that has been signed, in this example it is "Hi Joe, this is my signed message" right click and "Edit as HTML". In the image below I added a full stop after message:


After editing the text be sure to click again outside the edited text for the change to be written to the browser, then click the verification icon again and you will get the warning message, conversely if you then remove whatever you added (a full stop in this case) the message will be successfully verified.