560 leads from old list aren't dupes on new list?

Here's my dilemma... 

I've mailed to this Absentee list 5 times. My response rate has dropped off significantly. I just bought a new Absentee list and compared it to my old list (scrubbed from all responses so just those who haven't responded remain) and I kind of expected all but maybe 40-50 out of the 1300ish old list to be dupes on the new list... but to my surprise there were 560 that weren't on my new list that were still on the old list.  

Do I keep mailing to the 560 that aren't on my new list? or is that a good way of knowing those 560 are dead leads because they obviously didn't fit my list criteria like it used to 4-5 months ago due to selling, equity $ or %, absentee status, etc? 

If those 560 are really dead leads I'd hate to keep mailing to them :)