What exactly prevents you from sending messages?

When I try to send a message I get a pop up box that says Status: Connected to smtp.btinternet.com Progress: A solid green bar Then nothing happens and if I cancel I am then told "unable to send message"

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.

Application Basics

   Name: Thunderbird
   Version: 52.5.2
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20171221110448
   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, , (imap) imap.googlemail.com:993, SSL, passwordCleartext
     OUTGOING: , smtp.btinternet.com:587, plain, passwordCleartext, true

   account8:
     INCOMING: account8, , (imap) imap.gmail.com:993, SSL, OAuth2
     OUTGOING: , smtp.gmail.com:465, SSL, OAuth2, true

   account10:
     INCOMING: account10, , (imap) mail.btinternet.com:993, SSL, passwordCleartext
     OUTGOING: , smtp.btinternet.com:587, plain, passwordCleartext, true

 Crash Reports
   https://crash-stats.mozilla.com/report/index/bp-e4151180-166f-4544-b64a-a227d0171217 (17/12/2017)

 Extensions
   Lightning, 5.4.5.2, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   messagenotesplus, 1.5.0.0, true, messagenotesplus@mozilla.org

 Important Modified Preferences

   Name: Value

     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: 52.5.2
     font.internaluseonly.changed: false
     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
     gfx.crash-guard.glcontext.appVersion: 45.8.0
     gfx.crash-guard.glcontext.deviceID: 0x9904
     gfx.crash-guard.glcontext.driverVersion: 15.301.1901.0
     gfx.crash-guard.glcontext.gfx.driver-init.direct3d11-angle: true
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle: true
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-force-d3d11: false
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-force-warp: false
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-try-d3d11: true
     gfx.crash-guard.status.glcontext: 2
     gfx.direct3d.last_used_feature_level_idx: 0
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: cf939698-e763-4894-80a5-4be90cab821
     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: 1515935646
     places.history.expiration.transient_current_max_pages: 122334
     plugin.importedState: true

My btinternet.com account is about my least reliable. It fails to connect more often than it succeeds. The fact that it occasionally succeeds tells me there's nothing wrong with the settings. The fact that Thunderbird works well with all my other accounts tells me that Thunderbird is not, of itself, to blame.

Do yourself a favour and use another email provider.

Fascinating. But I use Mail on my ipad and also use Mail in Windows 10 and neither of these has any issue with my btinternet email address. The problem only manifests itself in Thunderbird

INCOMING: account3, , (imap) imap.googlemail.com:993, SSL, passwordCleartext OUTGOING: , smtp.btinternet.com:587, plain, passwordCleartext, true

Why is there a mix up between Google and BT servers?

Beyond that check with BT about the correct SMTP server settings. I somehow doubt your's are correct.

Try these settings:

mail.btinternet.com on port 465, SSL/TLS security, authentication = normal password, User name = full email address.

http://bt.custhelp.com/app/answers/detail/a_id/44917

I have checked with BT for the correct server settings (Port 465 SSL/TLS) and then I have checked in Thunderbird and these are the settings that are being used.

Check the Troubleshooting Information you posted. That's how your account is set up.

Just gone through the whole process again and outgoing server is definitely on Port 465

No no no, account 3 collects from Gmail but sends via BT. Set it to send via Gmail.

What you posted:

account3: OUTGOING: , smtp.btinternet.com:587, plain, passwordCleartext, true

account10: OUTGOING: , smtp.btinternet.com:587, plain, passwordCleartext, true

Thank you everyone for replying and for trying to help. I must just face it that I can no longer use Thunderbird, which, after so many years, is a great disappointment. As other email clients work and Thunderbird is the fly in the ointment I must just bite the bullet and use alternatives.

Just set it up properly. Please understand that you're trying to send via btinternet for a gmail account. Fix that first.