Here’s the e-mail trick Petraeus and Broadwell used to communicate
They wrote their "intimate messages" as draft e-mails in a shared Gmail account, according to the AP, allowing them to see one anothers' messages while leaving a much fainter data trail. When messages are sent and received, both accounts record the transmission as well as such metadata as the IP addresses on either end, something the two seemed to be seeking to avoid.
Petraeus and Broadwell apparently used a trick, known to terrorists and teen-agers alike, to conceal their email traffic, one of the law enforcement officials said.
Rather than transmitting emails to the other's inbox, they composed at least some messages and instead of transmitting them, left them in a draft folder or in an electronic "dropbox," the official said. Then the other person could log onto the same account and read the draft emails there. This avoids creating an email trail that is easier to trace.
Anon Note: You can also use the same email and capture emails from the sent folder if you are emailing to and from a censored country like China. There can be long delay's in between times sent and received this gets around, if you don't care that they are reading what you wrote.