How far in the past was the date? It's possible the clocks on the recipients mail servers are wrong.
EST daylight savings change was last night. If the reply was roughly an hour in the past then it sounds like someone has a computer/server that isn't patched up to date with the new EST daylight savings date changes that went into effect a couple years ago. If that's the issue the problem will resolve itself in a few weeks since their computer/server will be following the old EST daylight saving date change schedule.