People using SpyRecon, eBlaster, or other similar programs to spy on their spouse's email could be looking at serious jail time. An article form the Austin American-Statesman discussed recent cases that have transpired, even one in Austin. Basically, the reading of email without permission can be considered equivalent to illegal wiretapping. It is legal for parents to monitor their children's use of the Internet if the are a minor (this is presumably what the spy software is really for).
Unlawful interception of electronic communication is a second degree felony and holds a max jail time of 20 years. Now, typically people found guilty do not get the max sentence for any crime if plea bargain is reached...however, even going to jail for 5 years would not be fun.
So before you go looking around your partner's email without permission, think if it is worth it to spend a couple of years in jail if you get caught.