GPGrelay

GPGrelay is, as indicated by its name a local relaying server.
It works completely transparent for your Email-Client as well as for the remote Server.
Now, if you want to send emails encrypted, GPGrelay encrypts them and sends the encrypted mail to the SMTP-Server.

If you receive an encrypted mail, GPGrelay does the decryption for you - so your Email-Client never sees any encrypted mails, which is quite a nice feature when your Email-Client (like Outlook Express) is not capable of handling those mails.

But this also means, if an eavesdropper has access to your local environment, where Emails are stored decrypted, he can easily get access to them!