What is the best product or service to use for secure transmission of documents between clients and tax preparers, with information going both directions?

Does ProSeries offer that service, and if so, is there an easy and quick way to sign up for it within the software, and get it running?

What does it cost?

Are there other providers available that offer this service that preparers are commonly using?

Is there a way to simply acquire encryption for email transmittals, including attachments to those emails?  Or, would this effort only protect things I send to clients, but not things clients send to me?

I am not up on all the electronic products, which ones are best, or how to get them set up and running.  I have not needed this in the past, but I do need to look into it now, and fairly quickly.