Here's the screenshot showing the issue

Go to the Help menu. Select Troubleshooting Information Unchecked the box to include account names Click Copy to Clipboard Paste that info into a reply window here on the forum If you get a Too Much Info error delete all the printer info.

Airmail, Thanks, here's the info:

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

   account3:
     INCOMING: account3, , (imap) imap.gmx.com:993, SSL, passwordCleartext
     OUTGOING: , mail.gmx.com:465, SSL, passwordCleartext, true

   account10:
     INCOMING: account10, , (imap) imap.yandex.com:993, SSL, passwordCleartext
     OUTGOING: , smtp.yandex.com:465, SSL, passwordCleartext, true

   account11:
     INCOMING: account11, , (imap) imap.gmail.com:993, SSL, OAuth2
     OUTGOING: , smtp.gmail.com:465, SSL, OAuth2, true

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

 Crash Reports

 Extensions
   Lightning, 5.4.8, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     browser.cache.disk.capacity: 358400
     browser.cache.disk.filesystem_reported: 1
     extensions.lastAppVersion: 52.8.0
     font.internaluseonly.changed: false
     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: c1cc1fef-cc64-44b3-9e50-15e80a8fc34
     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: 1526738112
     places.history.expiration.transient_current_max_pages: 122334
     security.sandbox.content.tempDirSuffix: {88eb6883-3519-4260-9245-f8e6f2f93eb1}

 Graphics

     GPU #1
     Description: Intel(R) HD Graphics 620
     Vendor ID: 0x8086
     Device ID: 0x5916
     RAM: Unknown
     Drivers: igdumdim64 igd10iumd64 igd10iumd64 igd12umd64 igdumdim32 igd10iumd32 igd10iumd32 igd12umd32
     Driver Version: 23.20.16.4849
     Driver Date: 10-27-2017

     Features
     Direct2D: false
     DirectWrite: true (10.0.17134.1)
     WebGL Renderer: Google Inc. -- ANGLE (Intel(R) HD Graphics 620 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

I do not see anything obvious. Each account seems to have the matching SMTP server. Maybe someone will spot something I am not seeing.

Airmail. Thanks for taking a look, yes I too had checked the server settings and the configurations seemed fine. That's one reason why I was puzzled by this issue, and until its resolved I'm unable to use my emails accounts, Hope someone can assist in providing a solution.

Identities is the obvious thing to look at here, but normally I would expect additional identities with differing SMTP server to appear in the troubleshooting information as a second SMTP server line.

However in case my knowledge is not complete in this, please look at the manage identities button in each account to make sure that an additional identity is not configured on any of the account with the wrong SMTP server configured for the identity.

When you go into the compose window, click the drop down list for the from. IS there only one entry for the address listed. The ability to "change/edit " the from in the compose windows came with the last Thunderbird version. I am wondering if this might be in someway related to that. Please also confirm you are not using the "Customize from address" on the end of that drop down list to set the from address. That would definitely lead to this sort of issue I would think.

sfhowes, firstly thank you for so kindly offering advice.

I did not create an alias address with my GMX account.

As to my Yandex account, yes the Outgoing Server (SMTP) is correctly showing as configured to Yandex

Matt, Thanks for responding, I appreciate your kindness.

Having taken a look, following your suggestion, on the attached screenshot you can see the result. Hmmmm

UPDATE

So this morning my email account was suddenly working, able to send without the issues mentioned above. All very curious and makes me wonder if the problem was somehow a Thunderbird glitch? I'm hoping it will remain in working order, meanwhile I remain baffled why these obstacles arouse and what caused it. Will keep you posted.....

mogruith said

Will keep you posted.....

Please do.

Shall do and thanks for your advice,

Update....

Email account in Thunderbird was (without knowing why) suddenly working fine, this lasted only a few days as today when emailing the issue of not being able to connect to the server resurfaced (although at least this time the server address was correct)

" Sending of the message failed.The message could not be sent because the connection to Outgoing server (SMTP) smtp.yandex.com timed out. Try again."

I'm hoping this is a Yandex glitch and not Thunderbird....

...well having tried via Thunderbird to send an email from a GMX account I have that also failed to send due to connection failure to the server...I am now concluding the issue is very much linked to Thunderbird..Anyone got any ideas on what may be causing this?

Hi sfhowes, thanks for your thoughts on that. I am happy to try that out, however I wonder, if such timeouts are triggered by incompatible firewall/security surely that would not be an intermittent issue but apply consistently? What do you think?

...the problem with opening in Safe Mode is that it shuts down the internet connection, so not able to test if emails can be sent in TB

..thanks will give that a go...

Having opened in Safe-Mode Network and trying to send an email within TB the following message appears:

'Sending of the message failed. An error occurred while sending mail: Outgoing server (SMTP) smtp.yandex.com is unknown. The server may be incorrectly configured. Please verify that your Outgoing server (SMTP) settings are correct and try again.'

The server configuration though is correct, so that seems a little curious!

I also disabled both AV and Firewalls and was still not able to send email due to 'connecting to server' issue.

Thoughts?

All those configurations are set but still not connecting to the outgoing server, so I suddenly wondered....could it be the VPN being used. Anyway I disconnected from that and tried again, the emails were sent off! I repeated that and again once I reconnected to the VPN there was an error in connecting to the outgoing mail server. Soon as I disconnected the VPN the outgoing emails were sent. Question now is how to remain protected by my VPN yet still be able to send emails from TB? Any suggestions?