IMO, and this is my opinion only, I think there is a difference between 'snooping', and having duplicate copies of each and every correspondence sent to himself. Again, this is my opinion. Snooping is one thing, going to the trouble of setting up the roadrunner account to duplicate everything is a whole 'nother can of worms. IMO, that is a whole nother level of 'snooping'.
Not any trouble to set up. Would be the same (less than 1 minute) effort needed to setup your iPhone or BB to receive email in addition to receiving it on your PC. You set the option on the shadow account to leave emails on the POP server after downloading; it would show up in the POP server logs but not anywhere else.
Same with following the cell phone usage of a spouse. Usually about 6 months of all calls under the same account are available online. People having affairs or otherwise doing something secretive tend to use Trakphones and other prepaid phones and use text messaging rather than leaving an email trail.
None of this is secret. In fact, it is one of the staples of the publishing industry. How to detect cheating, how to cheat, how to protect yourself in a divorce or even how to "win" -- these articles are all over the popular press, mostly in pubs aimed at women. Just browse the magazines in your local grocery store.