Is there a way through either content search or Powershell that I can search my document libraries for documents containing email addresses. can I use regular expressions etc?
If you have a list of emails/a specific email domain that you are looking for yes you should be able to do that. Otherwise seems there are not much options. You can check out this too http://www.jwsecure.com/2008/09/24/new-security-tool-sharepoint-regex-search/