Just curious...why the need for the encryption for the emailing part? Is it an invoice from a shopping cart web app or such? if you absolutely have to then you have to..but if there's room to wiggle, i'd rethink the design instead of dealing with certificates, signing, encrypting, and securely sending emails. for example, bank doesn't send statements directly to customer. instead they just send an email letting the customer know the statement is available online, when the customer securely logs in. the only really (mostly) acceptable way for secure email is on a private encrypted network.