If this problem started last Friday then the problem is most likely that Suddenlink has stopped supporting SMTP server on port 587.

Resolution is to change the values in Thunderbird. Highlight the email account - click on "View Settings for this Account" - Edit SMTP Server. Change the Connection Security to SSL/TSL. Port should now be 465, connection security now SSL/TLS and authentication method now Normal Password. Press okay, press okay and retry.

Before posting, I verified that Suddenlink had not changed any thing, including smtp port 587 with their technician. We reviewed all smtp settings and they were correct. That being said, I couldn't resist trying what you said. I changed to SSL/TLS and it automatically chose port 465. Sadly, this did not work.

( Thank you tho.

re : I changed to SSL/TLS and it automatically chose port 465. But did you also select Authentication Method: 'Normal Password' ?

After changing the server settings for the Outgoing server and 'OK' everything, did you restart Thunderbird and then attempt to send?

Yes, I had selected "normal password', but I did not restart TB. so I just did, and it still doesn't work.

I'm getting frustrated. My send and receive works on all other apps on all devices. TB is the only one that stopped the send function. Nothing had changed in the last 7 months, everything was working fine. That is why I'm thinking an update must've changed or disabled something. If I can't get it working, I hate to say I will be forced to change email apps.

You are not the only person to have this sudden issue with Suddenlink, but in all other cases, the instructions have worked. As I'm typing this yet another person tried advice and it worked. eg: https://support.mozilla.org/en-US/questions/1267045?utm_campaign=questions-reply&utm_medium=email&utm_source=notification#answer-1245630

In outgoing server smtp, please make sure your 'usename' is the full email address.

Please post the following: In Thunderbird Help > Troubleshooting Information click on 'Copy text to clipboard' button Paste the info into this question in forum.

OK, here's what I figured out. Since I have 3 accounts in Suddenlink, each having their own outgoing server info, once I deleted all but the one I was working with, and made the changes above, it worked! Then I re-added the other two with your above settings and they also work. Thank you so much!

Requested Info: (probably not needed since it is working)

 Application Basics

   Name: Thunderbird
   Version: 60.8.0
   Build ID: 20190703133823

   Update Channel: release
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0
   OS: Windows_NT 10.0

   Multiprocess Windows: 0/0 (Disabled)
   Web Content Processes:
   Stylo: content = true (enabled by default), chrome = true (enabled by default)
   Enterprise Policies:
   Google Location Service Key: Missing
   Google SafeBrowsing Key: Missing
   Mozilla Location Service Key: Missing
   Safe Mode: false

 Mail and News Accounts
   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account3:
     INCOMING: account3, , (pop3) pop.suddenlink.net:110, plain, passwordCleartext
     OUTGOING: , smtp.suddenlink.net:465, SSL, passwordCleartext, true

   account4:
     INCOMING: account4, , (im) prpl-gtalk, plain, 0
     OUTGOING: , smtp.suddenlink.net:465, SSL, passwordCleartext, true

  
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I too had the same problem with Suddenlink. I changed connection security: SSL/TLS & Port Default to 465. It works for me! Thanks for the help! ( I didn't get much from Suddenlink)