Sebastien, thanks for the info, I'll try that.

Ronald, this is the specific line back we get:

Error: 421 4.7.0 [TSS04] Messages from xxx.xxx.xxx.xxx temporarily deferred due to user complaints - 4.16.55.1; see https://postmaster.verizonmedia.com/error-codes

Ronald,

No email blasts allowed, we have user limits set pretty strictly to enforce this, mostly user-to-user

Yes

Most, 300 or so domains, and we don't control DNS for all of them so getting DKIM and SPF correctly configured for them all isn't easy. All of our big senders have it configured.

Yes, we occasionally get them, but it almost feels like they need one setup for every single domain you host for it to work properly.

Ryan,

We haven't had this problem with AOL/Yahoo!/Verizon but we have had this problem with OpenSRS recently and it was a nightmare. Apparently there are a half dozen new RBLs that are all forks of UCEPROTECT (or at least built upon an outdated version of their database). Unlike UCEPROTECT that warned admins repeatedly not to use their L2 and L3 lists to Block because they contain entire ASNs, these new RBLs aren't giving the same warning and they are being used like a normal RBL for blocking purposes instead of just adding a score to a weight. (I don't have a list of them with me at the moment, but last I checked they weren't included on MXToolBox's RBL Checker, were all European, and two of the six didn't even have websites and two of them required that you pay to get delisted. They all seemed kind of sketchy, IMHO.). Alas, OpenSRS is using them and all of a sudden in 2020 we were listed on these half dozen RBLs for a single email sent to a Spamtrap in June 2007, which was over 13 years ago (and originating from a neighboring Class C on the same ASN)! Apparently Constant Contact is blocked by them for the same reason.

We thankfully have a Class B and 3 different Class C's that all are from entirely different ASNs so we were ultimately able to use an Outgoing IP in a different IP Range, but changing IPs in the same Class B (/22) wasn't enough to get email accepted. They were clearly blocking the entire ASN.

Best of luck to you. Best advice I have would be to give Sebastien's advice a try. It might not work but it's probably the best way to get someone's attention who might be able to help.

Everyones info has been helpful, got in touch with some postmasters at Verizon Media on that list and they revealed a little more info.

Thanks!