Digital signed mail- certificate issuing

[David Southwell]

Hi

Can anyone please tell me the simplest way I can issue my customers a means of 
digitally signing emails they transmit to us via our server. I need the 
chosen method to be compatible with most popular email clients and popular 
webmail services. 

Every customer has their identity and email addresses stored on our mysql 
database.

Essentially my target is , as far as possible, to ensure that emails 
purporting to come from my customers are indeed from them and noone else.

Running freebsd 6.1

postfix