How many Yahoo email customers receive encrypted emails? Since this is an automated process, I suppose Yahoo wouldn’t even notice if anyone did, they’d just scan the encrypted text and not find anything.
It is possible for a Yahoo email account to send/receive encrypted emails, but it would require a separate program that uses the IMAP or POP services at Yahoo.
Microsoft Outlook, Thunderbird, and other commonly available email clients support a standard known as S/MIME. But, if you are going to go through all that effort, you could use ANY email provider, including setting up your own on a virtual private server.