Comcast mail admin contact?

We are having trouble that seems to look like we are being throttled from one of our production nets to Comcast's pop3 service (mail.comcast.net). Service appears to work fine from other addresses in our network, just transactions from one of our more active production source IPs seems to progress like molasses or sometimes connections time out completely though we are not experiencing any packet loss so it looks like throttling of some sort. If there's someone from Comcast or someone else on the list who could point me to the proper contact for admin of that service, I'd be much obliged. It is having significant impact on some of their users who reach them via our services. G