Email Certificate (https://phas.ubc.ca/email-certificate)
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 do a web search for "personal email certificate" to find out who offers them.
Please be sure to backup your certificate.
Once you have the certificate backup done, you can install it in any email client you choose to use.