I would encourage everyone who has posted here to call tech support and report the problem as formally as possible (I know, I KNOW how unpleasant a task that is). One of the things my uncooperative tech rep told me was that it didn't count if only one customer reports something, no matter how many times. I would love to see some documented evidence (i.e., postings on this forum) that multiple people have reported this.
Thank you so much for that. I am using Thunderbird and my server name was already set to roadrunner but my authentication was set to "Transmitted Insecurely". I changed to :No Authentication" and now can send.
Thanks Carlab10 for your suggestions. I think they work for a number of people.
I use good old Eudora for email and found a solution that works
--Go into the "Tools" menu at tge top then click "Options"
--Under "Options" click on "Sending Mail"
--On the "Sending Mail" screen uncheck "Allow authentication" and change "Secure Sockets When Sending" to "Never"
At that point, you can send email.
By the way, I believe this is a scam by Spectrum to try to get you to upgrade to "Business Internet" for $100 a month. Don't fall for it. Call your local cable franchising authority (township, city or county) or your congressional rep. This tactic is illegal. Essentially, it's blackmail: Pay or we'll cut off your ability to send email.
I am using Xeams as a mail relay for my domain. On 9/27/2017 all my outgoing mail through mail.twc.com started failing suddenly with no changes on my side. I found the "sender rejected" error in the logs. Fortunately, I found today that removing the authentication credentials for mail.twc.com as suggested earlier seems to work. For the queued e-mails, I had to manually change the X-SMCustomAuth line at the top of each queue file to remove the credentials. Now all of my previously queued e-mails are starting to go out successfully.
I am using Thunderbird here. I found that the change to port 25 was not needed. I left it at 587 but turned off authentication and it worked fine. Better tech support here that contacting Spectrum.
I can't receive any emails from someone I've been using for YEARS!!! Above is all jibberish to me. What should I do TWC, Spectrum, or whatever you call yourselves these days?
I can confirm that removing authentication seems to bypass any 'sender' check by mail.twc.com smtp server and relay is allowed at least while on your twc internet connection. This solves my problem for using mail.twc.com as an smtp relay in my mail server (hMailServer) and sending from custom domains. Also allows forwarding all mail to my gmail account so long as the sender isn't using SPF.
We started experiencing "SENDER REJECTED AUP#I-1320" messages on both of our kc.rr.com email accounts on our home PC about Friday Oct 6 . Some sending problems but mostly receiving, with the senders calling us to tell us their emails to us were blocekd. It was intermittent and we could communicate amongs RR accounts, but not 3rd parties. I performed lots of updates and troubleshooting on my computer but nothing helped. My email accounts on-line at RR.com showed no problems and I received no messages from TW or RR or Spectrum.
I tried the solution referenced in these posts of changing the SMTP server settings from kc.rr.com to roadrunner.com, the Outlook validation test gave me an OK, and since then we have had no "sender rejected" errors on either account. Our SMTP server authentication was and still is NONE, and the outgoing port was and still is 25. I also changed the outgoing server from kc.rr.com to roadrunner.com. Good luck.