Include user's email with their post

Are you using V2 LiQL to get other info on the post already? You can just add author.email to the list of fields you are getting.

So, for example this LiQL query would get the author's login name and email along with the subject of the message.

SELECT subject, author.email, author.login FROM messages WHERE id = '1234'

and here is the full GET request:

https://[COMMUNITY DOMAIN]/api/2.0/search?q=SELECT subject, author.email, author.login FROM messages WHERE id = '1234'

encoded:

https://[COMMUNITY DOMAIN]/api/2.0/search?q=SELECT%20subject%2C%20author.email%2C%20author.login%20FROM%20messages%20WHERE%20id%20%3D%20%271234%27

I am more familiar with API V2 but it looks like you can get a variety of info about the author including email with this V1 call found here Community API v1 Reference - Khoros Developer Documentation

http://community.lithium.com/community-name/restapi/vc/messages/id/[id] /author

Also, keep in mind normally LiQL does not require authentication, but email is private info and would require authentication with the appropriate permissions.