By signing a message, you will be able to prove to the recipient that you are the actual author of the email (authenticity) and that the text has not been tampered with along its way from your computer to the recipient's inbox (integrity).
For more information, you can read the [http://www.bitcoinnotbombs.com/beginners-guide-to-pgp/ Digital Signatures] section on the Bitcoinbombs website and then a [https://securityinabox.org/en/guide/thunderbird/windows practical guide for your email client] from the Security in -a -box website.
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