Is anyone else seeing this error from AOL or know definitively what it is:

Connected to 152.163.0.68 but greeting failed./Remote host said: 421 
mtaig-aaj02.mx.aol.com Service unavailable - try again later
Connected to 64.12.91.195 but greeting failed./Remote host said: 421 
mtaig-mcb01.mx.aol.com Service unavailable - try again later

It’s not listed in the AOL error code page: 
https://postmaster.aol.com/error-codes <https://postmaster.aol.com/error-codes>

It used to be very occasionally, but for an IP that sends 100k-200k 
messages/day, it’s grown to ~17% of connection attempts on high volume days and 
~3-10% on lower volume days -- really picking up around October 11th. It does 
seem correlated with when the IP sends at a higher speed.

It also happened heavily for a couple of hours last night 9:45 - 11:45 US 
Central time; Thu Nov 17 03:45-05:45 UTC) on a number of IPs including IPs that 
hadn’t had much/any of this error before. Some IPs had ~100% and some had ~50% 
of delivery attempts with this error. Some low volume IPs didn’t have the error 
at all.

It spiked again to 100% of delivery attempts for a 5 minute window this morning 
on the high volume IP, and then returned to normal.

Does anyone else see this? Or does anyone know what this error code means for 
sure?

At first I thought AOL was having internal capacity issues, but I don’t want to 
assume that.

If this is a sign that AOL is unhappy with the email or throttling it, then I 
want to listen to that and be proactive. The IPs show a “good” reputation on 
AOL’s IP lookup page, but I’ve seen AOL throttle IPs that show “good” 
reputation.

There was a discussion on mailop October 13th about this error, but I wonder if 
this is a different issue?

Thank you,

David

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