Try to set 'Connection Security' to STARTTLS.

Sorry, that didn't work. Error message below.

Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/kb/safe-mode

Does the problem go away?

I'm afraid not - same error as before.

Attached is a screen shot of my SMTP settings, which appear to be correct.

Please post the server settings as per your email provider instructions.

Also, 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.

Email provider settings:

Username – this is your full email address Password – your chosen email password Incoming mail server IMAP or POP3: mail.uwclub.net Incoming port IMAP: 993 or POP3: 995 Outgoing mail SMTP server: mail.uwclub.net Outgoing port SMTP TLS: 587 or SSL: 465 Make sure that your security settings are set-up to enable TLS (or SSL if not available)

Troubleshooting information:

 Application Basics

   Name: Thunderbird
   Version: 52.6.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20180123185941
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

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

   account3:
     INCOMING: account3, , (pop3) mail.uwclub.net:995, SSL, passwordCleartext
     OUTGOING: , mail.uwclub.net:465, SSL, passwordCleartext, true

 Crash Reports

 Extensions
   Lightning, 5.4.6, false, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     accessibility.lastLoadDate: 1519753896
     browser.cache.disk.capacity: 358400
     browser.cache.disk.filesystem_reported: 1
     extensions.lastAppVersion: 52.6.0
     font.name.monospace.el: Consolas
     font.name.monospace.x-cyrillic: Consolas
     font.name.monospace.x-unicode: Consolas
     font.name.monospace.x-western: Consolas
     font.name.sans-serif.el: Calibri
     font.name.sans-serif.x-cyrillic: Calibri
     font.name.sans-serif.x-unicode: Calibri
     font.name.sans-serif.x-western: Calibri
     font.name.serif.el: Cambria
     font.name.serif.x-cyrillic: Cambria
     font.name.serif.x-unicode: Cambria
     font.name.serif.x-western: Cambria
     font.size.fixed.el: 14
     font.size.fixed.x-cyrillic: 14
     font.size.fixed.x-unicode: 14
     font.size.fixed.x-western: 14
     font.size.variable.el: 17
     font.size.variable.x-cyrillic: 17
     font.size.variable.x-unicode: 17
     font.size.variable.x-western: 17
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: dc514ba4-65f0-45fe-af80-34697e727fd
     media.gmp.storage.version.observed: 1
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1519679804
     places.history.expiration.transient_current_max_pages: 122334
     security.sandbox.content.tempDirSuffix: {5eb0fdca-25db-4018-b53f-984934c0a354}

 Graphics

     GPU #1
     Description: Intel(R) HD Graphics 630
     Vendor ID: 0x8086
     Device ID: 0x5912
     RAM: Unknown
     Drivers: igdumdim64 igd10iumd64 igd10iumd64 igd12umd64 igdumdim32 igd10iumd32 igd10iumd32 igd12umd32
     Driver Version: 21.20.16.4526
     Driver Date: 9-23-2016

     Features
     Direct2D: false
     DirectWrite: true (10.0.16299.15)
     WebGL Renderer: Google Inc. -- ANGLE (Intel(R) HD Graphics 630 Direct3D9Ex vs_3_0 ps_3_0) -- OpenGL ES 2.0 (ANGLE 2.1.0.2a250c8a0e15)

     AzureCanvasBackend: skia
     AzureCanvasAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: skia

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.13.1
     4.13.1

     NSS
     3.28.6
     3.28.6

     NSS Util
     3.28.6
     3.28.6

     NSS SSL
     3.28.6
     3.28.6

     NSS S/MIME
     3.28.6
     3.28.6

Your account settings look good. Can you confirm you did start Windows in safe mode?

Yes, that's correct.

Since you mentioned Norton Security in your OP, double check it isn't blocking Thunderbird.

I turned off the antivirus and firewall in Norton, then tried sending an email from Thunderbird and it worked. Then I gradually turned on the firewall and antivirus functions one at a time, sending an email between each change and it still worked. Finally I returned to the point where all the firewall and antivirus functions were running again and it still worked. So I'm not sure why or how it happened, but it seemed like turning Norton off and back on again cleared the problem.

When your problem is fixed, can you mark the topic as 'Solved' please? Thank you.

Many thanks! We got there in the end.