"Let's remember here that the users are (hopefully) using the stored emails to conduct the business the company exists to conduct."
Therein lies a potentially big problem...e-mails should be regarded as transitory, not as formal records
Suppose the ever popular Alice and Bob work for separate companies, but they figure out a plan to make one or both of their businesses better.
Alice sends Bob the revised process description.
To cover their backs they both send a copy to their respective bosses,Agatha and Bill.
Agatha and Bill realise that this has commercial implications, so send copies to their respective commercial, legal, QA departments. Finance dept. may also get involved
There are now (at least) 10 - 12 copies of the document floating around.
In quite short order, Alice and Bob may make minor changes, the commercial and legal folks will also want to communicate with their counterparts and change things ...
If you believe that all 12 copies of the document will remain aligned, I have a bridge to sell you :-)
When recovering from a SharePoint/Exchange outage, the choice of 'point of truth' will be stressful for the (innocent and not involved) IT techie [who will pick the 'wrong' one from the point of view of many].
I know that using Git, SharePoint ... would ease the issue but, in my experience, getting management types to use them properly (or even at all) is an exercise in futility and frustration when simply attaching stuff to e-mails is easy to do.
If, by some miracle, things turn out well, Agatha and Bill celebrate and get their bonuses; if normality ensues, Alice and Bob suffer career limiting reviews for 'not following process'. Either way Microsoft gets its licence income.