There are multiple threads on this forum about this. Here is what I found that is the root cause of the problem.
I found yesterday that I also was experiencing this same issue. This morning after some searching I was able to resolve in my mail server. What Spectrum has changed is that they are looking for the MAIL FROM Reverse-Path or Return-Path to be is the Spectrum login and not another email address. This is a function described in the SMTP RFC 5321 (https://tools.ietf.org/html/rfc5321). I found in Mercury Mail that I can set a Forced SMTP Sender with that login and it works. It does not affect the reply-to or whom the email came from.
I primarily use Thunderbird as my client and could not find a way to specify this in the setting to use the internal SMTP engine. Not sure about other clients, some may support this setting. I would say that Spectrum needs to change back whatever setting they recently changed, as it will be a large problem for many users that have email addresses other than their Spectrum address.
I hope that this helps to clarify the issue in more detail.
For me, it ended up being an authentication method issue. I could send email on this same account from my other devices, but not from my desktop computer.
I use Seamonkey mail on my desktop. In my Mail & Newsgroups Account Settings, I changed my outgoing SMTP server Authentication method to No authentication and now it works fine.
Hope this helps someone else.
I was having trouble since Friday. I tried everyone's configuration solutions and what worked for me was a combination of two solutions: changing the smtp server to smtp-server.roadrunner.com and also changing the outgoing server authentication to none. I left the outgoing server port at 25 and bingo, I finally had success. Hope this helps others!
This worked for me as well (changing server and removing authentication). THANK YOU for this!!!
I'm currently running OS X 10.9.5 with Apple Mail (located in NYC). I just had an incredibly frustrating conversation with Spectrum Tier 3 tech support with Melanie B., who was polite but unfortunately didn't have the slightest idea what she was talking about. She said it categorically wasn't their server's fault (it obviously is), and that I should call Apple to ask why Mail suddenly started received rejection messages from the POP server after 3+ years without a configuration change. AND she said that there were no reports of problems on the issue (I offered to read her the URL to this page, but she refused to take it). The most annoying part of this is that I spoke to a tech support person a few days ago who IMMEDIATELY acknowledged the problem and said that they were working on it.
At any rate, changing the server name, and setting the authentication method to "none" worked for me. I suspect that this solution is temporary; removing authentication for outgoing mail sounds like a quick-and-dirty fix while the IT people try to make a more permanent fix. So be prepared for things to break again in the near future (just my guess).
It seems like they have changed the setup and allow unauthenicated SMTP on Port 25 as long as you are on their network. That would the device being used is sending from a connection provided by Spectrum. If you do this in a mobile device, it would work while on the network and would fail when you are running cellular or on another WiFi network.
Removal of the authentication info fixed the issue for me as well.
I echo the concerns if this is a permanent fix and will we have to figure it out ourselves again if not.
It is concerning that their tech support was clueless many times and for those of us that rely on our email for work, this is serious when it is down for so long.
SKova makes an important point that removing SMTP authentication will only work if you are connected to the Internet via Spectrum's network. So this solution, while helpful for some of us in some scenarios, is not complete or satisfactory.
Had this issue since Wednesday. Tried many options, even so called tech support couldn't fix it. The authentication did not work for me at all. Using thunderbird as well. Tried calling spectrum, they refuse to send me to an American tech support. Trying again now. we already canceled our phone and TV with them