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 - Win8 https://support.microsoft.com/en-us/help/17076/windows-8-startup-settings-safe-mode - Win7 https://support.microsoft.com/en-us/help/17419/windows-7-advanced-startup-options-safe-mode#start-computer-safe-mode=windows-7

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

Does the problem go away?

All sorts of comninations to check]

1). Boot windows safe, use Tbird normal: long (ie >140KB) email fails with "The message could not be sent because the connection to Outgoing server (SMTP) mail.btinternet.com timed out. Try again. "

2). Boot windows safe, use Tbird normal: short (ie <140KB) email works

3). Boot windows safe, use Tbird safe: long (ie >140KB) email fails with "The message could not be sent because the connection to Outgoing server (SMTP) mail.btinternet.com timed out. Try again. "

4). Boot windows safe, use Tbird safe: short (ie <140KB) email works

Conclusion Windows safe mode or not - irrelevant TBird safe mode or not - irrelevant Message length <>140KB - relevanr

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
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.

How do you connect to the internet? Wireless, router etc.

There are some old support issues around old hardware. so have their been any changes in your connection hardware? especially using old hardware.

Thanks for your interest:

Hardware: Computer is elderly (~5years), but the only modification in that period was a new HDD about 6months ago. OS (64-bit Win7 home premium) and all applications (including Tbird) live on an SSD

Internet Connection I live in the sticks, and use a satellite connection, but everything is hardwired, so I have desktop -> ethernet -> draytekRouter -> ethernet -> satelliteModem

TroubleShooting info

 Application Basics

   Name: Thunderbird
   Version: 52.2.1
   User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20170622064432
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) mail.btinternet.com:110, plain, passwordCleartext
     OUTGOING: , mail.btinternet.com:25, plain, passwordCleartext, true

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

 Crash Reports
   https://crash-stats.mozilla.com/report/index/bp-4cf672ef-af4a-47f5-97ae-0819a2160911 (11/09/2016)

 Extensions
   Lightning, 5.4.2.1, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     accessibility.typeaheadfind.flashBar: 0
     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: 52.2.1
     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: Times New Roman
     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.direct3d.last_used_feature_level_idx: 0
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: dc792187-7816-4467-8fe9-6b3d0d34290
     media.gmp.storage.version.observed: 1
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1501591508
     places.history.expiration.transient_current_max_pages: 44949
     plugin.importedState: true
     plugins.update.notifyUser: true
     print.printer_HP_Officejet_Pro_8600_(Network).print_bgcolor: false
     print.printer_HP_Officejet_Pro_8600_(Network).print_bgimages: false
     print.printer_HP_Officejet_Pro_8600_(Network).print_colorspace:
     print.printer_HP_Officejet_Pro_8600_(Network).print_command:
     print.printer_HP_Officejet_Pro_8600_(Network).print_downloadfonts: false
     print.printer_HP_Officejet_Pro_8600_(Network).print_duplex: 1852796448
     print.printer_HP_Officejet_Pro_8600_(Network).print_edge_bottom: 0
     print.printer_HP_Officejet_Pro_8600_(Network).print_edge_left: 0
     print.printer_HP_Officejet_Pro_8600_(Network).print_edge_right: 0
     print.printer_HP_Officejet_Pro_8600_(Network).print_edge_top: 0
     print.printer_HP_Officejet_Pro_8600_(Network).print_evenpages: true
     print.printer_HP_Officejet_Pro_8600_(Network).print_footercenter:
     print.printer_HP_Officejet_Pro_8600_(Network).print_footerleft: &PT
     print.printer_HP_Officejet_Pro_8600_(Network).print_footerright: &D
     print.printer_HP_Officejet_Pro_8600_(Network).print_headercenter:
     print.printer_HP_Officejet_Pro_8600_(Network).print_headerleft: &T
     print.printer_HP_Officejet_Pro_8600_(Network).print_headerright: &U
     print.printer_HP_Officejet_Pro_8600_(Network).print_in_color: true
     print.printer_HP_Officejet_Pro_8600_(Network).print_margin_bottom: 0.5
     print.printer_HP_Officejet_Pro_8600_(Network).print_margin_left: 0.5
     print.printer_HP_Officejet_Pro_8600_(Network).print_margin_right: 0.5
     print.printer_HP_Officejet_Pro_8600_(Network).print_margin_top: 0.5
     print.printer_HP_Officejet_Pro_8600_(Network).print_oddpages: true
     print.printer_HP_Officejet_Pro_8600_(Network).print_orientation: 0
     print.printer_HP_Officejet_Pro_8600_(Network).print_page_delay: 50
     print.printer_HP_Officejet_Pro_8600_(Network).print_paper_data: 9
     print.printer_HP_Officejet_Pro_8600_(Network).print_paper_height: 11.00
     print.printer_HP_Officejet_Pro_8600_(Network).print_paper_name:
     print.printer_HP_Officejet_Pro_8600_(Network).print_paper_size_type: 0
     print.printer_HP_Officejet_Pro_8600_(Network).print_paper_size_unit: 1
     print.printer_HP_Officejet_Pro_8600_(Network).print_paper_width: 8.50
     print.printer_HP_Officejet_Pro_8600_(Network).print_plex_name:
     print.printer_HP_Officejet_Pro_8600_(Network).print_resolution: 1701344288
     print.printer_HP_Officejet_Pro_8600_(Network).print_resolution_name:
     print.printer_HP_Officejet_Pro_8600_(Network).print_reversed: false
     print.printer_HP_Officejet_Pro_8600_(Network).print_scaling: 1.00
     print.printer_HP_Officejet_Pro_8600_(Network).print_shrink_to_fit: true
     print.printer_HP_Officejet_Pro_8600_(Network).print_to_file: false
     print.printer_HP_Officejet_Pro_8600_(Network).print_unwriteable_margin_bottom: 0
     print.printer_HP_Officejet_Pro_8600_(Network).print_unwriteable_margin_left: 0
     print.printer_HP_Officejet_Pro_8600_(Network).print_unwriteable_margin_right: 0
     print.printer_HP_Officejet_Pro_8600_(Network).print_unwriteable_margin_top: 0
     privacy.sanitize.timeSpan: 4
     security.sandbox.content.tempDirSuffix: {25e1bc54-8be9-4294-a0c1-9bb72fe65ec0}

 Graphics

     GPU #1
     Description: NVIDIA GeForce GTX 680
     Vendor ID: 0x10de
     Device ID: 0x1180
     RAM: 2048
     Drivers: nvd3dumx,nvwgf2umx,nvwgf2umx nvd3dum,nvwgf2um,nvwgf2um
     Driver Version: 22.21.13.8233
     Driver Date: 5-17-2017

     Features
     Direct2D: false
     DirectWrite: true (6.2.9200.22164)
     WebGL Renderer: Google Inc. -- ANGLE (NVIDIA GeForce GTX 680 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
     3.28.5
     3.28.5

     NSS Util
     3.28.5
     3.28.5

     NSS SSL
     3.28.5
     3.28.5

     NSS S/MIME
     3.28.5
     3.28.5

Further info

I just configured Outlook as an email client for the same account, using the same 140kB signature card.

"short" email (ie no signature card): Outlook works "long" email (ie with 140kB signature card) Outlook works

No sure what this proves except that it seems unlikely, there is any issue with my Email provider (BT)

Mysterious solution?

After a bit of googling on this problem, one suggestion was to change my account settings to

Authentication Method: Encrypted Password Connection Security: SSL/TLS

This failed with a slightly different error message, so I reset these to where they were originally, ie

Authentication Method: Password transmitted insecurely Connection Security: None

Somehow this seems to have "cleared" the problem. Since all I did was change two settings, then change them back again, I have no idea why this should "fix" anything

Solution would be to use the setting BTinternet actually provide instead of just trying random things in the hope they work. Even if the do work, they are not supported by BTInternet and further random failures are quite probable.

They provide the settings here http://bt.custhelp.com/app/answers/detail/a_id/44917/~/what-are-the-settings-for-outgoing-and-incoming-bt-email-servers%3F

So the connection settings for SMTP should be

Mail Server: mail.btinternet.com Port: 465 (this may not be automatically populated on selecting SSL, so you'll need to check) SSL Encryption: SSL (but not STARTTLS) Authentication: Normal Password Username: your email address including the @btinternet or @btopenworld.com part Password: your btinternet or btopenworld password

Perhaps the most important thing there is to change the port from 25. It is a wonder they accept your connection at all, and anti virus program these days often simply block the port as something used by malware.

Your incoming setting also need work.

Incoming Mail Server: mail.btinternet.com Port: 995 (this should be automatically populated by selecting the SSL encryption) Connection Encryption: SSL (but not STARTTLS) Username: your email address including the @btinternet or @btopenworld.com part Password: your btinternet or btopenworld password

I was trying not to complicate the basic issue.

Until about three(?) years ago BT actually subcontracted their email service to Yahoo Mail. When BT announced that they were bringing their mail service back "in-house", they claimed that existing customers (like me) didn't need to change anything. So I changed nothing and the settings I have posted are those relevant for this "original" Yahoo/BT account., which BT must still be supporting as some sort of "legacy" configuration. I imagine BT adopted this policy because of the chaos which would ensue if they required all of their existing customer base to change their email settings. Their helplines would probably have gone in to meltdown

These settings have been working quite happily for about the last ten years, first with BT/Yahoo as the provide and for the last three(?) years, with BT as the provider

However all "new" BT customers from that time were provided with a "new" list of settings

As a matter of interest, when I mentioned earlier that I had set up Outlook as a client on the same account as part of the diagnostic process, I let it "autodetect" all settings, it came up with BT's "more recent" configuration - and it worked. I then manually configured it to use the SAME (ie legacy) settings as TB was/is using - and it still worked. From this I concluded that my current problem was unrelated to configuration settings.

So when you state

"Solution would be to use the setting BTinternet actually provide instead of just trying random things in the hope they work. Even if the do work, they are not supported by BTInternet and further random failures are quite probable.",

this is incorrect - they may be "legacy" settings, but they are still supported by BT. They just don't advertise the fact (and certainly don't tell any "new" customers about them.

I can think of a couple of reasons why I should update to BT's current recommended configuration,

1) Improved security 2) Since these are "legacy" settings, they are probably going to break at some point in the future