In order for us to better investigate this issue you need to add a little more non-personal information.

Please add the troubleshooting information to your post. To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help);
• Choose Troubleshooting Information;
• Clear the check box "Include account names";
• Use the button Copy to clipboard to select all;
• Paste this in your post.

Please post back with this information and any other questions you might have.

I would comment that "encrypted passwords", whilst valid, is an unusual configuration, so I would want to know if the setting has changed and if it is indeed required by your email provider.

Hi Zenos,

As requested:

 Application Basics

   Name: Thunderbird
   Version: 52.8.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20180516145259
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account1:
     INCOMING: account1, , (none) Local Folders, plain, passwordCleartext

   account12:
     INCOMING: account12, , (imap) imap.gmail.com:993, SSL, OAuth2
     OUTGOING: , smtp.gmail.com:465, SSL, OAuth2, true

   account13:
     INCOMING: account13, , (imap) imap.gmail.com:993, SSL, OAuth2
     OUTGOING: , smtp.gmail.com:465, SSL, OAuth2, true

   account14:
     INCOMING: account14, , (pop3) pop3.talktalk.net:110, plain, passwordEncrypted
     OUTGOING: , smtp.talktalk.net:587, plain, passwordCleartext, true

   account15:
     INCOMING: account15, , (pop3) pop3.talktalk.net:110, plain, passwordEncrypted
     OUTGOING: , smtp.talktalk.net:587, plain, passwordCleartext, true

 Crash Reports

 Extensions
   CompactHeader, 2.1.5, true, {58D4392A-842E-11DE-B51A-C7B855D89593}
   Zindus, 0.8.42, true, {ad7d8a66-253b-11dc-977c-000c29a3126e}
   Test Pilot for Thunderbird, 1.3.10, false, tbtestpilot@labs.mozilla.com

 Important Modified Preferences

   Name: Value

     accessibility.lastLoadDate: 1528113521
     browser.cache.disk.capacity: 358400
     browser.cache.disk.filesystem_reported: 1
     browser.cache.disk.smart_size_cached_value: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     dom.apps.reset-permissions: true
     extensions.lastAppVersion: 52.8.0
     font.name.monospace.el: Consolas
     font.name.monospace.tr: Consolas
     font.name.monospace.x-baltic: Consolas
     font.name.monospace.x-central-euro: 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.tr: Calibri
     font.name.sans-serif.x-baltic: Calibri
     font.name.sans-serif.x-central-euro: 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.tr: Cambria
     font.name.serif.x-baltic: Cambria
     font.name.serif.x-central-euro: 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.tr: 14
     font.size.fixed.x-baltic: 14
     font.size.fixed.x-central-euro: 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.tr: 17
     font.size.variable.x-baltic: 17
     font.size.variable.x-central-euro: 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: bb24eb24-f350-4e7a-9331-91800333b10
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     mailnews.database.global.views.global.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     media.gmp.storage.version.observed: 1
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1528195911
     places.history.expiration.transient_current_max_pages: 122334
     plugin.importedState: true
     security.sandbox.content.tempDirSuffix: {f2597319-afcb-464d-bcc4-e1b7885c6949}

 Graphics

     GPU #1
     Description: Intel(R) HD Graphics 5300
     Vendor ID: 0x8086
     Device ID: 0x161e
     RAM: Unknown
     Drivers: igdumdim64 igd10iumd64 igd10iumd64 igd12umd64 igdumdim32 igd10iumd32 igd10iumd32 igd12umd32
     Driver Version: 20.19.15.4835
     Driver Date: 10-16-2017

     Features
     Direct2D: false
     DirectWrite: true (10.0.17134.1)
     WebGL Renderer: Google Inc. -- ANGLE (Intel(R) HD Graphics 5300 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

Thank you.

Your settings don't tally with those shown here:

https://help2.talktalk.co.uk/email-settings-imap-pop3

I am not surprised that your settings for "passwordEncrypted" provoke errors.

It's possible that talktalk have updated their settings. Yours are using practices that used to be common but have fallen out of favour. I'd recommend you change them to use SSL and STARTTLS, along with the accompanying ports.

Well, that solved it, but no idea how it has been working up until today! Thanks for your help.