Encrypting E-mail

I have been asked to implement e-mail encryption on the e-mails people on my domain send to our clients. The CTO wants select people on my network to be able to send encrypted e-mails with minimal set-up and no required technical knowledge needed by users. I have been the Sys Admin for my company for 5 years but we’ve never needed this so I haven’t got any experience with it. Can anyone give me a starting point.I have started testing with Verisign Digital ID’s but that seems to require some work by the end user which I need to try and avoid.
Any help would be appreciated.

Thanks