The only way all traces could disappear is a multi-staged, highly coordinated effort, over time, across multiple people and organizations.
I won't detail all of the technical and process intricacies... but every time a user or system administrator performs any activity on any server - a log entry is created. Additional meta-data is captured with the event so that events or problems can be identified, reversed, or diagnosed. So, who, or whatever (user or system process) deleted email data - it would be in the syslog files. If the system policy deleted the emails, we would know what user set up the system policy to illegally delete data, mandated by law, to be retained. Busted either way.
So the original data, plus multiple incremental and full backups of the original data, system logs, incremental and full backups of system logs, offsite backups of said data, offsite backups of system logs, and offsite backup logs of the facility that is hosting all of the backup data - would have to be meticulously wiped!
Log files get backed up too. There are process compliance reasons for log files to get saved - to see who logged into a system, accessed data, changed the system policies, etc. Log files are not very large compared to the size of the process or data elements they track, so they're very valuable and cost effective to keep for security and audit reasons.
So the bottom line is that the emails exist elsewhere and anyone who tries to alter system internals - like deleting data, altering backup policies, editing log files, deleting traces of accessing system data - it's all captured for various reasons and backed up offsite. Deleting offsite data would require a conspiracy with the offsite facility across multiple people and systems - because all offsite maintenance is tracked by user and system, logged, and backed up - just like the original system data.
We should get the metadata from the White House and Senate for the time period requested.
We need to look for links back to Lerner, or the other six IRS employees in trouble.
Obama cannot claim executive privilage, as we are only looking at Metadata.
The NSA has told us, over and over and again, that the NSA does not read e-mail content, they simply look at metadata.