This Saturday, the machine that hosts dev.null.org was due to be booted out of its present location and moved to a new location. On Friday afternoon, I received notice that the router for the new location hadn't arrived and would do so on Tuesday or Wednesday, and thus that everything hosted there would be offline until then. Rather annoying, but since I get my hosting for free from friends, not really anything one can complain about. So I spent Friday afternoon scrambling around, finding a new temporary virtual mail host and arranging secondary DNS servers.
Only later I found out that one of my existing secondaries wasn't working properly and the other still had outdated MX information. So I changed my domain record to remove the dysfunctional DNS servers, leaving only the primary (which is offline) and a new secondary a friend set up for me on Friday evening.
Later, after my mail still hadn't started arriving, I found out that my registrar's web form seems to have "corrected" the name of the new secondary DNS server, prepending the 'ns." I left off. Mind you, I left it off precisely because the server is named foo.bar, not ns.foo.bar, which does not exist, and hence mail wasn't being delivered. Anyway, it should be changed now, and some mail has started trickling through. (Actually, one message has trickled through, a few hours ago. Though that's probably to do with DNS propagation being what it is.)
So, if you've tried to email me over the past few days, this is why it hasn't gotten through.