Exchange 2013 introduced another mechanism to help ensure that e-mail gets delivered to users: Safety Net. This feature creates a copy of all inbound messages cross site (by default, but configurable). Exchange stores the messages in yet another .edb file and yet another storage place on the server.
You can move the location of the files, but you must follow the directions and there is no option during setup to designate where to store safety net information. The default install is a subfolder of the location where you install the Exchange files:
By default, all safety net information is stored for 2 days. This is a rolling 2 days of messages getting purged. In other words, at 1pm, the messages from 48 hours ago are deleted. At 1:01pm, 48 hours previous, those messages are deleted and so on.
Safey Net hold time is configurable; however, there is an upper limit of 24,855 days plus a little bit. This means if you set this to the maximum value, you’ll be able to store just over 68 years’ worth of e-mail messages.
PowerShell does remind you to make sure this value is equal or greater than your lagged copies, and since lagged copies have a maximum limit of 14 days, this much larger value is safe to implement.
Just a reminder, if you do plan to keep 68 years of data, please make sure you have enough disk space available.