A week ago, I suddenly could not send e-mail from home, where I use Outlook 2016 (although I could receive it). Today, I fixed the problem by revising the account setup as given by HisSon's Community posting. In the process, I changed the in/outbound servers to mail.twc.com and set the outbound mail server port to 587.
I called Spectrum yesterday and after a few transfers I got someone who acknowledged the problem. Best he could do was give me a trouble report number (TRB7267705). Not sure how to look this up to get a status, so perhaps someone else knows.
I asked for an ETA for the fix and he could not give me one.
None of the solutions here helped me. Avast tells me that the certificate has expired and it won't let me connect. I tried to use Outlook's "fix" procedure, even to the point of installing a new certificate (using their promots) but to no avail.
had the same problem when I updated my iphone to the new ios11. they had to add a patch to fix this. I think they need to do the same for Mac. don't think it's a Spectrum problem but a software problem