Email certificates provide the strongest levels of confidentiality and security for your electronic communications by allowing you to digitally sign and encrypt your mail and attachments. Encryption means that only your intended recipient will be able to read the mail while digitally signing allows them to confirm you as the sender and verify the message was not tampered with en route.
For personal and home users, you can sign up for a free email certificate from Comodo (https://www.comodo.com/home/email-security/free-email-certificate.php)
Once you have installed the email certificate, here is how you can backup the certificate: https://www.instantssl.com/ssl-certificate-support/server_faq/ssl-email…
Once you have the certificate backup, you can install it in any email client you choose to use:
Microsoft Outlook: https://www.comodo.com/support/products/email_certs/outlook.php
Mozilla Thunderbird: https://www.comodo.com/support/products/email_certs/thunderbird.php