SMTP error from remote mail server after end of data:
521 5.2.1 : AOL will not accept delivery of this message.
Mike also mentioned his IP reputation is good, when he checks at AOL so he doesn’t understand why mail is being blocked.
I think the big clue is after the end of data and would look at the full content of the mail, particularly domains and URLs, to identify is triggering the block.
In the SMTP transaction there are only a few places the ISP can stop the transaction and each spot tells us different things about why the ISP is rejecting the message.
A block after connection is a block either against the IP address or against the domain in the rDNS of the IP. IPs with no rDNS or generic DNS can also be blocked here. Blocks here do happen, but many recipients will let the SMTP transaction continue.
A block after HELO/EHLO is often a block against the domain in the HELO/EHLO or against a particular HELO/EHLO. Malware and bots often have distinctive HELO/EHLO patterns and it’s common for those kinds of senders to be blocked at this point.
After Mail From
A block after Mail From is often directed at the domain in the bounce string. Some senders do check to make sure the domain has a MX and will block if it doesn’t. Blocks don’t happen here very often.
After RCPT To
Blocks here are not always spam related. Most of the delivery failures at this point have to do with non-existent addresses.
Blocks after data mean the ISP has actually seen the full content of the email. If a block comes after DATA the full content of the message including the recipient and their permission status should be evaluated as part of the determination about what is triggering the block.
Using when the rejection happened is an important part of understanding why a block happened. For instance, if a block happens before DATA, you know that content isn’t relevant, because the ISP never saw the content. If a block happened before Mail From: you know it’s the IP address reputation or configuration. If a block happened after DATA you know you need to look at the whole message.