In addition to being a Spectrum (residential) customer, I administer a small online forum. Several of the forum members use a Spectrum / Roadrunner email account. Starting on, or about July 5th, 2019, I have started having issues with the Spectrum email servers rejecting (inbound) messages from my email server to Spectrum user accounts.
An example of the current error that I am receiving is: Jul 18 14:06:20 telvos postfix/smtp: C2EF71F975: to=<email@example.com>, relay=dnvrco-cmedge01.email.rr.com[220.127.116.11]:25, delay=5.9, delays=0.01/0.01/2.2/3.6, dsn=5.2.0, status=bounced (host dnvrco-cmedge01.email.rr.com[18.104.22.168] said: 552 5.2.0 ESMTP server temporarily unabailable AUP#I-1510 (in reply to end of DATA command))
In other words, server dnvrco-cmedge01.email.rr.com is rejecting messages with code 5.2.0 / AUP#-1510.
From what I can tell code 5.2.0 sometimes refers to attachements, though there are none.
The email server is my hosted (Linode) domain. According to MX Toolbox It has:
Valid forward and reverse DNS A records.
Valid SPF record
Valid DKIM signature.
Valid DMARC record.
It is NOT listed on the black lists.
Has a reputation score of 99 (out of 100) according to other testing services.
It does NOT send UBM (unsolicited Bulk Mail) commonly called SPAM. Users must opt in and select to receive desired notifications.
In short, there is no valid reason why the server should be rejecting messages from this host.
Update. I made a slight changed today and may have found / corrected the issue, unless something else was resolved on the backend without my knowledge.
The email server hosts a couple of different virtual domains. The email in question was being sent as 'from' the forum email address and was signed using the DKIM signature for that account, whose key is available by text record for the domain. I also have a DKIM signature for my personal domain (email) which is how the email server identifies itself.
I changed the DKIM for the forum to sign emails as the personal server, and the test messages went through. Apparently the DKIM should correspond to the DNS record of the IP address, not the 'sender'.