Emails to comcast.net addresses throttled
I run an email server behind a static IP address associated with a Comcast Business Internet account. This server has been running behind this address for over 6 years. When sending emails to comcast.net addresses lately, they are being delayed, and I'm seeing the following message returned from Comcast's server:
421 4.2.0 x.x.x.x throttled - try again later. please see http://postmaster.comcast.net/smtp-error-codes.php#rl000001
Where x.x.x.x is our static IP address.
Our SenderScore is 0. We are a small business, and do not send any type of marketing emails. According to the link above, we should be able to send 1,200 messages per hour with a Sender Score of 0. We don't even send that many emails in a month! This issues usually occurs when an email is sent to the same group of people, with around 4 having comcast.net addresses. Emails to comcast.net addresses are not sent every day, and may only be sent a few times per day.
We do not have any issues sending emails to any other domains, including yahoo.com, outlook.com and gmail.com, the issue is only with comcast.net addresses.
I suspect the issue is with the flawed Sender Score system. We have a low score because we don't have the volume of a large corporation, not because we send spam. We shouldn't be throttled for that reason!
I've called the Comcast Customer Security Assurance 4 times now, but they have not been very helpful. Can anyone else from Comcast assist with getting our IP address whitelisted, so we can send emails to comcast.net addresses without an issue?