How can I technically verify\* that the e-mail I'm sending from Gmail to Yahoo is encrypted all the way from Gmail's servers to Yahoo's servers? I know that it's advised to have encryption between the mail service provider and the client's desktop, e.g. using IMAPS or SMTPS, and when using a web interface, HTTPS. But if an e-mail is sent from e.g. the web interface, then that e-mail could be sent in "plain text" format through the mail relay servers until it reaches its destination mail service provider. 

\* besides reading the corresponding FAQs