To send encrypted email, you need a "key pair". One is a private key and should ONLY be known to you and reside on your computer. The other is a public key, which is the key that others have to decipher your encrypted email.
This is an article I wrote a while back explaining the system within Linux, but the operating system is irrelevant. The principal is the same for Windows or MAC.
Unless Yahoo has done something specific to block encrypted emails, the transmitting service you use should be irrelevant.