The most secure way to send information might well be sending encrypted messages written in secret ink with a homing pigeon.
If skilled people
really want to read your (unencrypted) mails, there are probably ways to intercept them, even when you use your own mail server.
If you use a third-party service, just be sure to read their privacy policy.
However, Hushmail is indeed supposed to be extremely secure, but I haven't read their privacy policy. If you're looking to send extremely sensitive information, your best bet might be to encrypt the mails beforehand with e.g.
OpenPGP, paste the encrypted message in Hushmail (which you access via an anonymous proxy of course) and send it.
That way, only the recipient will be able to decrypt your mail.