I've read the available similar questions on serverfault but I haven't quite found a definite answer to the security aspect of it - hence here's my question:

I'm administrator of an office working with tax data and we want to start using certificate-based eMail encryption with our clients. Considering the prices for issued certificates by VeriSign & Co I was wondering if we couldn't issue the necessary certificates with a certificate authority of our own.

I realize that they do not offer the trust hierarchy that commercial certificates do but I don't see why we would need that. Most of our clients have small businesses and only 20% of them even exchange data with us via email. So if we were to issue certificates for those 20% and our employees, that would enable us to use encrypted emails. Of course they would have to trust our certificate authority and thus once receive our public root certificate. But if we would hand them out to them (or install it) personally, they'd know that it really is our certificate.

Is thery a huge security risk that I am missing here? As long as nobody has access to our certificate authority server nobody should be able to interfere with security, right? And the client certificates would be generated and handed out by us, as well...

Please advise me if I am making an error in judgement here and thank you in advance.