You can encrypt those emails with PGP, the best encryption there is.
A good email client can also learn to automatically encrypt when you send to a specific email, so you can send to a specific alias (with a plus or subdomain aliasing scheme).
But then will your diary service work right with it? Honestly, you should use a mail server that isn't from an ad company. And then if you also want to layer encryption on top, feel free.
(Also, calling PGP "the best encryption there is" might be a bit of hyperbole.)
A good email client can also learn to automatically encrypt when you send to a specific email, so you can send to a specific alias (with a plus or subdomain aliasing scheme).