Interesting issue. I think it depends on largely whether you're looking at it form a moral or a legal standpoint.
A few things to add to the conversation:
1) Work email and home email are two different animals. At work, you are given the email as a benefit, and likely a tool, of your employment. You use it at (and for) work and your boss/hr dept/computer guy has your password (or can override it). There is no "personal" component to it. Often, you are restricted in your use of email at work by company policy and/or you are given policy that indicates your email is subject to review (with penalties for misuse that are as serious as termination).
2) Especially if she had the passwords sitting next to the computer (as a poster noted), the wife has no reasonable expectation of privacy it the email. Spouse or not, if she took no method to conceal the password and hold the email as her own, he can look. It's as if she left a letter sitting on the desk. If she hadn't left the password there, there is still a low chance of expectation of privacy against someone that lives there, etc.
3) I don't think that it matters at all that they are spouses. Spouses can commit crimes against each other and are held to the same criminal standard as any other two people would be. There's no heightened amount of responsibility or privacy because they are spouses. Maybe what people are thinking of is spousal priviledge. A spouse can claim a marital prividelge and choose (the key word being choose) not to testify against the other spouse, but that is not something that has to be done or can be forced in any way. It's a privilege (asserted to protect the sanctitiy of the relationship) but is the person's decision. That never prevents a spouse from claiming crime agfainst her by her husband (and vice versa).
4) With new law, and with more technology, come the tests within the legal system. Sure, this may not be what the law was designed for but certainly, she has enough to start a proceeding and test the theories.
5) The better question regarding the evidence, though the point may be moot if she is already divorced, is whether the Judge will allow it to be admitted as evidence in the divorce matter given the method by which it was obtained.