Safe-mail Secure Drop-Box allows you to easily send protected e-mail messages to any e-mail recipient, even if the recipient is not a Safe-mail user or equipped with a digital certificate!
The outgoing message is stored in a Drop-Box while a notification is forwarded to the recipient non-secure address. The recipient can access the protected message using a password which is taken from the recipient's contact card in the sender's address book.

To setup the defaults and use this function goto Preferences --> Security --> Mail security.
To read more click here