There are really three issues here: 1) policies need to be created to determine what needs to be kept and what can safely be deleted, 2) good spam filtering tools need to be implemented to make sure that spam doesn't get archived, and 3) good archiving tools need to be deployed to make sure that your data gets stored properly and can be accessed.

Many companies have found that their archiving solutions either were not scalable enough to index all of the data they need to preserve and/or that searches take too long. That's why it's important to spec a system properly based on anticipated long term storage requirements and to make sure that the search capabilities won't result in superfluous data being produced.

Also, most data does not need to be kept for a decade or longer. Some content can safely be deleted after much shorter periods, although legal counsel (and regulators in some cases) will be able to advise on retention periods for specific data types.