Hard Links for Copies
In our company, we use readonly public folders for departmental in/out mails, and read/write for custom archiving. As practically all mails get archived at least once, this leads to lots of wasted space.
When possible (folders on same drive), add support for and give the option to enable "create hard links for copied files". This would not only reduce space, but will probably also increase performance, due to fewer actual data = more unique content cached.
Thank you for your suggestion, it will be considered for future versions.