My Email Server And Roadrunner - Where To Turn

i run a very small hosting company (about a dozen clients) on a windows 2003 server with Helm and the latest version of Smartermail (www.smartertools.com) for my email server.

overall it works well but i have been experiencing problems lately when users on my email server try to send email to users at roadrunner (@nycap.rr.com). the messages always get delivered but sometimes they take hours to get delivered.

when i look in my mail server logs i keep seeing:
"RSP: 421 #4.4.5 Too many connections to this host.
CMD: QUIT"

so basically the message gets rejected by the roadrunner mail server and then when the server tries again to send the message it goes through. although sometimes it takes several attempts.

i have gone back and forth with roadrunner about this. according to their pages it looks like i am being affected by their inbound rate limiting policy but they SWEAR that there is nothing on their end preventing me from getting my messages through to their server. i have tried calling them, and emailing them at several different numbers and addresses and they all give me some BS answer.

my clients are really unhappy with the fact that their messages sometimes take hours to deliver, and i really don't know where to turn to get this resolved.

i don't have this problem sending messages to any other provider or domain, just roadrunner and unfortunately most of my clients communicate regularly with roadrunner users.