I am a mail administrator for the domain ccbchurch.com that sends roughly 10-15k emails daily to comcast subscribers (and roughly 1 million emails daily to all providers) for communications between our customers (we are a SaaS company). Starting Friday Jul 19 around 6am MDT we saw an increase in deferrals on all of our outbound mail servers coming from mx*.comcast.net indicating we're being throttled. We are also experiencing intermittent connection issues over port 25 even with telnet. This was the first time we've ever received such a message. We haven't changed our sending behavior, our sender score reputations are excellent (99 on all sending servers), we aggressively sanitize and prune our customer's recipient lists, and I believe our SPF and DKIM records are configured correctly. We aren't experiencing problems with any other providers. I have tried calling in to get help and have yet to speak with someone who understands even what we do. Might you be able to help me understand what's happening and what we need to do to fix it?
Here are some samples from our logs:
@emillerccb This is a newer system for our platform. We altered some tuning variables this morning, and hope it helps legitimate senders as yourself deliver properly. I'll DM you with some other suggestions that might help.