I think that taking a hard initial stance and being willing to go flexible on it (usually) works nicely because it encourages them to have to talk to me about their late work and not just hand it in whenever, but actually commit to "I will have this to you by this date".

In his case, I gave him a hard deadline that he agreed to - but the issue wasn't that the two papers in question were late. It's that he never got them to me at all. Really, his argument (if he continues to pursue this) will come down to one of two things:

a)Student claiming (falsely, it appears) that I had some problem receiving email on my end, even though he was successfully corresponding with me via email just the day before

or

b)Student claiming that his unsuccessful attempt (because he put the wrong address on the "to:" line) to send me an email should be close enough.

If the school administration ends up weighing in on this, I will reasonably accept the criticism that I need to hold to some harder lines to avoid stuff like this. That's fair. I won't accept any implication that I mishandled work that he submitted on time, because the screenshots that he pretended were evidence actually suggest the opposite - that he sent them to the wrong address. In short, "a" is a lie and I can't accept it. "B" would be pretty weak and risks exposing opening up a Pandora's box of a lot of ambiguity at each semester's end for all professors, not just me. But if they said "he tried. grade his stuff." then it actually wouldn't be the worst shit sandwich I've eaten for a job, and so I'd eat this one.