the archiving software commonly used is a client application that is installed or exists on either an MTA routing server or an MX gateway. This intercepts every message passing through it and decides whether to archive it or not based on the administrators selections. I’ve configured the software in the past, I know how it works and it does indeed matter what is selected as well as how long the archives are stored before they are purged. You can create policies for global use or smaller groups of individuals and have customized archiving rules applied to the given groups or even individual users. There may be lower-end applications out there that do not give you these options, but I don’t know who would use something like that.
That is another level which has nothing to do with whether the servers themselves are backed up.