can't send e-mail

Are you using Port 465?

See if they've gone to something like this...

 Many eMail providers now require a separate App Password different from your eMail password…

Then you may need Ways to securely access AOL Mail…

https://help.aol.com/articles/allow-apps-that-use-less-secure-sign-in

Or it may just need an App Password...

https://support.google.com/mail/answer/185833?hl=en-GB

Yahoo has gone to this too...

https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html

Yahoo App password…

Generate and manage third-party app passwords

Some older, third-party email apps (that do not use our Yahoo branded sign in page) require you to enter a single password for login credentials. To access your Yahoo Mail account on these apps, you'll need to generate and use an app password. An app password is a long, randomly generated code that gives a non-Yahoo app permission to access your Yahoo account. You’ll only need to provide this code once to sign in to your third-party email app.

https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html

Using app passwords with apps that don't support two-step verification (microsoft.com)

In your email app's Preferences is stored your user ID and passwords for sending and receiving emails. Apparently yours somehow has gotten corrupted. Open preferences and carefully check the stored information. Be sure that it matches what the email server is expecting to receive. If this doesn't fix it I would delete the account in the app and set up a new one.

To see if it's your provider, What's my ip...

http://www.whatismyipaddress.com/

Start with these three, check SpamCop or SpamHaus to see if your IP is there...

http://spamcop.net/bl.shtml

http://www.spamhaus.org/lookup.lasso

http://www.spamhaus.org/zen/

Sometimes an ISP will bloc a whole other ISP too, if it's the source of too much SPAM.

In a Browser does Login TWC & Roadrunner RR Email | Spectrum Webmail work?

Open the email app. Then down below create a Local Folder and label it "saved emails". Next drag your emails down into that folder. They will be saved there. Then you can delete and then create a new account.

Glad to hear it's fixed, take care. :)

I am suddenly unable to send mail. I can receive mail fine. For sending I get the following message:the sender address was rejected by the server mail.twc.com. The server response was: <> sender rejected. I tied to edit the SMTP server list but the server list does not identify IMAP or SMTP servers and when I try to add mail.twc.com as an SMTP server, the software just deletes. (so, presumably the Apple mail app still can't find it). How do I fix?

[Personal Information Edited by Moderator]

I believe Ronasara has you covered. :)