Most of us use Exchange Server and Outlook for our day to day corporate email systems. But there is an almost infinite variety of mail systems, protocols, options, preferences and behaviors.
And, if you wanted to make sure the email you send doesn't end up on the corporate email server, what do you do. You send and receive web mail.
Web Mail is mail sent and received though an email provider like yahoo, gmail, hotmail, msn or comcast. The mail is generally viewed through a browser like Internet Explorer or Firefox. In general, this mail is only maintained on the server's database. However, web mail can often be recovered from unallocated space or HTML files stored in the internet history folders. Like Exchange email, webmail may have headers, metadata and attachments.
Here's a relevant case study:
The Hiring Conspiracy