encryption of emails

["Walhalla" <walhalla@w-4.de>]

hi,
has, or does anybody know, an email-gateway, that encrypts emails 
automatically?
for example the whole mailexchange between several branches of a 
company? maybe a kind of "black-/whitelist" - all mails to 
users/domains on a list will be encrypted - the rest will be sent 
unencrypted?

it is important, that this works automated, serversided and no user 
has to do (forget ;) something!

and: is it possible to store incoming emails encrypted (postfix - 
Maildir), that f.e. no user with local access can read mails of another 
user on the server? when polling this encrypted emails, they should 
be decrypted automatically!

thx for any ideas & tips!

jens