I wanted to add that, after these old previews have appeared while in lock screen, they also show up in my notification centre as supposedly new emails. However, I should clarify that they don't then reappear in my inbox. So it's resending the alert, but not resending the actual mail itself.
It's just happened again with another mail from Monday (three days ago) so here's a screenshot of my notification centre (the white gaps are obviously where I've cut out the content of my emails).
The top email is the most recent one I've received.
The second one down is from Monday. It's not the second most recent one I've received (there have been at least 50 emails in between them) and I had already read and deleted it.