It is my guess that, if it is true that this was a hoax, Foley did not know it and responded with salacious e-mail. So he had to resign because of what he wrote, not what he received.
My understanding of the time is this: 1 - There is a report that Foley exchanged emails with an underage former page...EMAIL that seemed a little too friendly.
2 - Foley almost immediately resigned...then checked into alcoholic rehab.
3 - LATER ABC reports AIM messages which are somewhat more explicit. AIM stands for AOL Instant Message...these are different from EMAIL.
4 - Now it seems that the AIMs were a prank and that the former page Jordon Edmonds was over 18 at the time the AIMs supposedly occurred and they were actually a prank to goad Foley into typing something explicit...
Point is Foley resigned due to #1, the reasonably innoquous emails...not the post resignation apparently more explicit AIMs AND the now apparently NOT UNDERAGE correspondent.
Why did he resign...shame, embarrassment AND/OR yet to break info.