Throttling (RL000010 / RL000003) on mail - need help

We're a small HOA management company in California. Our homeowners with comcast.net addresses get our email hours late, sometimes more than a day, while every other provider gets it within a minute.

The deferrals read "451 4.2.0 Throttled" (RL000010) and "421 4.2.0 77.32.148.25 Throttled" (RL000003).

Our mail is DKIM-signed by hoaqualitymgmt.com and hoaqm.com (our domains), and SPF and DMARC (p=reject) pass. We email only our own homeowners: about 60 comcast.net addresses, a few times a month. Could someone at Xfinity help with our domains' reputation? I can send timestamps by DM.