In most cases, I'd say that if you see an email address in a question or answer, it should probably be edited out. The goal of Stack Overflow is to create a resource of knowledge to help people for years to come, and this means the focus is squarely on questions and answers.
If I post a question and then ask people to email me the answers, this doesn't help future visitors. If I'm a spammer posting an email address so I can sell you my service, this creates a lot of noise that detracts from excellent Q&A. If I post my email address, at minimum, I'm opening the door for spammers to obtain my email address.
I can't really imagine a case where a person should put his or her contact information directly in a post. It's noise, and it should be removed.
With that said, for special circumstances, one could leave a comment pointing someone to his or her user profile as a place to leave contact information, that way the body of the post is left unmolested by noise. Furthermore, the comments can be deleted when they've served their purpose.
Lastly, if the purpose of the email address is to use a signature, this is explicitly mentioned in the FAQ on Use of Signatures:
Please don’t use signatures or taglines in your posts, or they will be removed.
Every post you make is already “signed” with your standard user card, which links directly back to your user page.
Your user page belongs to you — fill it with information about your interests, links to stuff you’ve worked on, or whatever else you like!