AOL's
Postmaster Website has information on how to ask AOL to accept your mail, if it isn't coming from a trusted source yet. They generally only accept large volumes of mail from known-good trusted IP addresses; like most recipients, they don't care about SPF or DKIM until they've decided to accept a message.
The site also has a list of the error code explanations:
Quote:
421 DYN:T1
The IP address you are sending from has been temporarily rate limited due to lack of whitelisting, unexpected changes in volume, or poor IP reputation.
So, most likely, the IP address you're sending from isn't
whitelisted, and it suddenly started sending a lot more mail than it used to. The quick and easy way to handle this is to do the migration over the span of a few months; start routing a couple deliveries per day via the new server, then a couple more, etc, until it's all good. But I suppose the horse is already out of the barn on this one. At least it's a 4xx error, so it'll keep retrying until it goes through (or bounces).
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