mailspam@rcn.com exists for the sole purpose of allowing RCN customers (including Erols customers) to report junk email. If you've received junk email and all you want to do is report it, mailspam@rcn.com is the place to send it.abuse@rcn.com exists as a complaint address for The Rest of the Internet(tm). If RCN customers are being naughty, folks from other ISPs can send their reports to abuse@rcn.com. This address is also for RCN customers when they have questions that need answering, and which are not answered by this website.
Incidentally, as of this writing, "rcn.com" and "erols.com" are the same thing. I've gotten a couple emails from folks complaining that I should have announced the change from "mailspam@erols.com" to "mailspam@rcn.com". In actual fact, there has been no *substantive* change. Merely a notational one. mailspam@erols.com and mailspam@rcn.com both work and both go to exactly the same place. Same goes for abuse@erols.com and abuse@rcn.com. (And since they both go to the same place, please don't send one email to both addresses since we end up with two copies of the email when you do that.)