Please post your Troubleshooting Information. At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information. Press the Copy text to clipboard button and paste the information into your reply.

I don't understand the above instructions. Which Thunderbird window are you referring to. I don't have a menu button you describe. I am using Edge

Which Thunderbird window are you referring to.

How many Thunderbird windows do you have?

I don't have a menu button you describe.

Please post a screenshot of the main Thunderbird window. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

I am using Edge

What has this got to do with your question?

Sorry I was using a Theme on T Bird which doesn't show the menu you described. Now in Safe mode

Sending of the message failed. The Outgoing server (SMTP) smtp.cox.net does not seem to support encrypted passwords. If you just set up the account, try changing the 'Authentication method' in 'Account settings | Outgoing server (SMTP)' to 'Normal password'.

So, let's see your Troubleshooting Information, please.

Did you manage to find "SMTP (Outgoing Server)" settings when checking? Many users see the settings for incoming and think that's all there is.

Is this what you need?

Application Basics

   Name: Thunderbird
   Version: 45.5.1
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20161130024037
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) pop.cox.net:995, SSL, passwordCleartext
     OUTGOING: , smtp.cox.net:587, alwaysSTARTTLS, passwordEncrypted, true

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account3:
     INCOMING: account3, , (pop3) pop.cox.net:995, SSL, passwordCleartext
     OUTGOING: , smtp.cox.net:587, alwaysSTARTTLS, passwordEncrypted, true

 Crash Reports

 Extensions
   Theme Font & Size Changer, 50.0, true, {f69e22c7-bc50-414a-9269-0f5c344cd94c}

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Server settings are for normal passwords. Nothing I can do. Up to Mozilla I guess

OUTGOING: , smtp.cox.net:587, alwaysSTARTTLS, passwordEncrypted

Looks like it's up to you to fix your authentication setting.

OK thanks. Do I contact my ip people or is this something I do within the app??

This page at cox suggests that for sending you need either 465/SSL or 587/TLS, but you have 587/alwaysSTARTTLS. That wouldn't immediately seem to be a problem, but keep it in mind.

But you also have passwordEncrypted and that seems to be specifically what their server is objecting to.

Open Account Settings, go down to the last account in the list, "Outgoing Server (SMTP)", select your cox setting(s), click "Edit" and change Authentication Method from "Encrypted password" to "Normal password".

You should be able to do this yourself.

At the top right of the Thunderbird window, click the menu button > Options > Account Settings.

'SMTP (Outgoing Server)' is at the bottom of the left pane. Adjust 'Authentication method' as per your email provider's instructions.

If in doubt contact your 'ip people'.

Many thanks to you and Christ1 for the time you gave me. Problem solved!!!