According to internet search this has been a problem for several people. The error message is from Spectrum server. It may be that the server thinks you are not the real sender for that email address, possibly because you travelled.

https://forums.timewarnercable.com/t5/Email/4-days-still-can-t-send-email/td-p/141621 https://www.linuxquestions.org/questions/linux-server-73/sendmail-suddenly-stopped-being-able-to-send-4175614619/

Post info on server settings, so the forum knows ezxactly what you are currently using. In thunderbird Help > Troubleshooting Information click on 'Copy text to clipboard' button In this forum question, right click in a Post a Reply text box and select 'Paste' Edit/Remove all info on fonts and printers, but nothing else.

Have you tried using a different IP address?

Do you have a problem when you send a new write message or when it is a reply to email or both?

When you are trying to send, are those emails going from the 'Outbox' because you had previously used 'Send Later' or are you creating email and clicking on 'Send' to send now.

re:One guy ended up wiping out a lot of email in my laptop inbox. Did they say why the 'Inbox' because you said the issue was when sending not receiving?

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   account2:
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   account3:
     INCOMING: account3, , (imap) mail.twc.com:143, plain, passwordCleartext
     OUTGOING: , mail.twc.com:587, alwaysSTARTTLS, none, true

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   https://crash-stats.mozilla.com/report/index/bp-9292e08e-659f-434c-9ecb-97e112151225 (12/24/15)
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I receive the same message whether using a new form or replying. Yes, I have tried a new IP address - still away from home - but same result.

Alert An error occurred while sending mail: Outgoing server (SMTP) error. The server responded: Service not available, closing transmission channel.

Your problem is your mail provider. You will be connecting to the internet from a non TWC service, (hotel wifi etc) so they will not let you send mail. Connect with a TWC service and you will again be able to send mail.

Matt, You may be correct because I have had a similar problem with my iPhone - cannot send email from anywhere but home - which is frustrating.

  However, this 5-year-old MacBook Pro and sent and received email from all over the USA and in Asia and Europe. This inability to send email from the road is a new development that first cropped up on this trip.
 My gut tells me it is something else.

MK

Here are the recommended setting for TWC. https://www.timewarnercable.com/en/support/internet/step-by-step/twc-mail.html You need to change he IMAP port. The port for SMTP is correct but you have None set for password instead of Normal Password.

The settings for Spectrum are mail.twc.com on port 995 (POP) or 993 (IMAP) and 587 (SMTP). All connection security settings should be None, authentication is 'password, transmitted insecurely', and User name = full email address.

If you can't send when you're not on the Spectrum network, it's probably because other networks won't allow non-secure connections to Spectrum. But first enter the settings that at least are correct for the Spectrum network.

If you wish to send from your Spectrum address in TB when you're on a non-Spectrum network, you could e.g. add a gmail account to TB, add the Spectrum address to your gmail account, then set the Spectrum account to send through the gmail smtp in TB. Otherwise, send via Spectrum webmail.

re : OUTGOING: , mail.twc.com:587, alwaysSTARTTLS, none, true

I'm agreeing with sfhowes advice.

Info: https://www.timewarnercable.com/en/support/faqs/faqs-internet/e-mailacco/incoming-outgoing-server-addresses.html

IMAP settings Account Type: IMAP Outgoing Mail Server (SMTP): mail.twc.com port: 587 Outbound User Name: Enter full email address johndoe@[division].rr.com johndoe@roadrunner.com johndoe@twc.com

Inbound & Outbound Password: Email Password (same as Web Mail) SMTP Server Requires Authentication: Checked

There is no mention anything about SSL/TLS or STARTTLS and Using either of these would require 'Authentication Method:' 'Normal Password'.

In your case 'Connection Security' would be 'None' Authentication Method: 'password, transmitted insecurely'

Copied from info...

• Note: If you are able to receive, but not send, emails while on your mobile device, you'll need to use your mobile carrier's outgoing SMTP server and port.

Basically you cannot use this server for sending if you are not at home.