It's like anything Frank - if it's that sensitive that you don't want anyone other than the recipient to be able to read it then you need to find a way you're happy with. You could meet in person or encrypt a message and send it.

The reality is that electronic and phone communications can be intercepted and it's up to you to decide how secure the mechanism you're using is.

Sure the PM system on many forums is not particularly secure, but I'm comfortable enough that I'm not exchanging information that is sensitive enough to worry about. Also I'm pretty sure that the main people that would have access to the content of my PMs apart from the recipient, the mods, have far better things to do than to read through my messages