Hushmail, cyber-rights, elitefitness, ziplip are all compatible. They all use PGP encryption, and as long as the public keys are available, they are compatible and secure. PGP providers are VERY secure (if you passphrase is secure). The advantage to PGP encryption is that if other eyes try to snoop into your mail, it is not "economical" to decrypt (again ... based on how secure your passphrase is). Not even the companies that host PGP based email can snoop. What I mean by "economical" is that the more secure your passphrase is (based on bits on entropy), the more expensive and time consuming it becomes for interested parties to crack the encryption. Read the diceware link below.
Keptprivate is NOT secure. Keptprivate does use SSL encryption from the email server to your desktop, which can inhibit packet sniffers, but the email can still be easily read from the server.
Assume any other email provider (yahoo, msn, hotmail) is NOT secure. BTW, operamail is NOT secure. If there is a link "Forgot your password?" anywhere, the system is NOT SECURE.
KEY THINGS TO REMEMBER
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1. Do not accept unencrypted email (requires paid subscription with some providers).
2. Never send email unencrypted.
3. Your passphrase determines how strong the encryption REALLY is.
http://world.std.com/~reinhold/diceware.html
4. Do not lose or forget your passphrase or you are screwed. It can not be recovered.
Basic PGP Information
http://www.pgpi.org/doc/pgpintro/