Doesn't hugely concern me it's been "left out".

a) It's always been this way.
b) There are workarounds.
c) Note what Sebastians says.

When we get to an email provider situation that would be the time to consider implementing this functionality.

Of course we could delay 5.1 by another couple of months or so to fulfill your specific request (I don't think that would be reasonable).
We know there is demand for it..... but there are other priorities.

These hosters then do not understand mail security if their mail server trusts anything coming from the web server. This means any user hosting with them can run a script (using any scripting languages or in the case of DNN, use a module which sends out emails) to send out spam and pretend they are someone else. They can't log or audit the mail transactions because no username/password is being authenticated. All they can see is that they are coming from the web server.

When spam is reported, the mail server could be blacklisted by some public blacklists and then thousands of receiver mail servers in the world who subscribe to these lists, and they should, will refuse to accept any messages from the host. This is a serious issue.