Nevertheless do not go over 50 many ISP will start to be doubtful about your intentions if you go over that number. I do not know your application But also check if there is "timer" something like a pause in between batch Try to make it a bit smaller. Anyway 13000/50=260 even with 5 sec pause it might take anything in between 35 min to 1 hour. You might consider placing a call to your ISP and verify that they are OK with it.
i'll call my isp, that's a good i dea, i never would have thought of that...i was hoping to send in batches of 500 - it'll be for the monthly newsletter, i'm not motivated enough to bother with more than that lololol