PG, just about any e-mail system is open to interception and has vulnerabilities. Unless it is a closed system, at some point the transmission is interceptable. All encryption is breakable as well. It's just highly improbable that someone will go to that much trouble to intercept non-targeted e-mail. Basically, that person would be spamming himself since he can't determine what the targeted user is recieving or sending in his e-mail.
No, more likely e-mail interception would be targeted at a POI like a Government Official or highly placed corporate executive. Then you could be reasonably sure of the ROI for the work you have done to intercept the e-mail.
Just my opinion though. People have done stupider things I am sure.
-Skater