Something I have found is that sometimes what I would have hoped to have been auto-saved hasn't been - either an auto-save has not been done, or what has been auto-saved has been overwritten (for some reason or another...). Or that I just find/access it. So I would welcome the ability to manually (deliberately) save a draft, by clicking button.
So perhaps this should be a cue for a larger review of drafts, and how/when they are saved, accessed and (eventually) deleted.
My thoughts as a starter:
Start composing message (new/reply)
'Current' version auto saved as draft periodically, or on navigating away from page. Should these by the same? Should multiple generations be kept?
When message is sent, auto-saved drafts are deleted.
Go back to compose.
Latest auto-saved version is restored, to be completed...
Then as normal flow.
'Manually saved' draft:
Click 'save' button - draft saved. This would also be the 'latest' draft (until another auto-save is done), for the 'resumed' flow case above.
But that resumed flow should not automatically delete the manually saved draft, if has been automatically loaded.
If composition of the message continues (after manual saving), autosaving will not overwrite the manually saved version, and eventual sending will delete autosaved, but not the manually saved version.
Subsequent manual saving should not just overwrite the original manually saved version.
Possibly there should be an option on sending or manually saving to delete (or not) old manually saved drafts?
There should (somewhere) be a menu listing saved drafts - including, at the end (?) any auto-saved draft(s) with options to
i) delete draft
ii) resume composing the draft - with the ability (somehow, and documented, if not by separate options) to resume the draft, or keep it (long term) as a 'model'
Hope this makes sense to people - just my four bits worth.