https://support.mozilla.org/kb/cannot-send-messages

I tried various configuration for smtp server with port setting of 587, 465, 25 with and without security and authentication etc but nothing seems to work.

There is no point trying arbitrary combinations of server settings. Use the settings as per your email provider's instructions.

I can send emails using web portal so there seems to be no issue in email server.

Webmail infrastructure is independent from POP/IMAP/SMTP servers. Therefore, if webmail works, this isn't an indication that there are no other problems.

I had to completely uninstall and reinstall the Thunderbird few months ago when same problem occurred but this time it is not working too.

You've been wasting your time. Profile files are stored separately from the Thunderbird program files. The program files are static and do not change (except when you upgrade the Thunderbird application version). Because we store profile and program files separately, you can uninstall Thunderbird without losing your messages and settings, and if something goes wrong with a Thunderbird update your information will still be available. It also means that you don't have to reinstall Thunderbird in order to delete your information or troubleshoot a problem.

Thanks for pointing out some facts regarding emails servers. The client was working fine few days ago and suddenly it stopped working both for incoming and outgoing email and that's why I reinstalled the program. Thunderbird once reinstalled started fresh and tried to automatically find the email configuration for me but it failed. So, I manually configured the imap and smtp server, but only emails could be pulled from the server but still the outgoing mail not working. I have tried smtp setting without any authentication or password too otherwise the default is 587 port with STARTTLS and normal password ( not working, of course).

There is only Windows defender security in my system which keeps updating by itself. Do you think it is blocking the smtp? How to diagnose this smtp blocked issue?  It is really painful not able to send email through Thunderbird. Thanks  again for your help.

Thunderbird does not remove either your old mail or your previous settings when you use the standard uninstall. So lets start with what you did to prevent that fail safe from saving your data for you? One of those uninstaller applications perhaps? I see a lot of folk here that have user recuva to turn a bad situation into something worse.

You still have not mentioned what mail provider is giving you grief. So really there is no information in this thread on which to base any constructive feedback.

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.

Please also add the result of these troubleshooting steps to your post

• Does Thunderbird work in TB Safe mode (see Thunderbird Safe Mode)?

Thanks Matt for your help. This problem of inability to send email really baffles me and is really inconvenient for me. I have tried Thunderbird Safe mode too but I still could not send email in this mode.

As per your suggestion here's the troubleshooting information:

 Application Basics

   Name: Thunderbird
   Version: 60.2.1
   Build ID: 20180930223627

   Update Channel: release
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1
   OS: Windows_NT 10.0

   Multiprocess Windows: 0/0 (Disabled)
   Web Content Processes:
   Stylo: content = true (enabled by default), chrome = true (enabled by default)
   Enterprise Policies:
   Google Key: Missing
   Mozilla Location Service Key: Missing
   Safe Mode: false

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

   account6:
     INCOMING: account6, , (imap) imap.ntc.net.np:993, SSL, passwordCleartext
     OUTGOING: , smtp.ntc.net.np:587, alwaysSTARTTLS, passwordCleartext, true

 Crash Reports for the Last 3 Days

 All Crash Reports

 Extensions

     Name
     Version
     Enabled
     ID

     Lightning
     6.2.2.1
     true
     {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Security Software

   Type: Name

     Antivirus: Windows Defender Antivirus
     Antispyware: Windows Defender Antivirus
     Firewall: Windows Firewall

 Graphics

     Features
     Compositing: Basic
     Asynchronous Pan/Zoom: none
     WebGL 1 Driver WSI Info: EGL_VENDOR: Google Inc. (adapter LUID: 000000000001a495)

EGL_VERSION: 1.4 (ANGLE 2.1.0.7edc21933ef1) EGL_EXTENSIONS: EGL_EXT_create_context_robustness EGL_ANGLE_d3d_share_handle_client_buffer EGL_ANGLE_d3d_texture_client_buffer EGL_ANGLE_surface_d3d_texture_2d_share_handle EGL_ANGLE_query_surface_pointer EGL_ANGLE_window_fixed_size EGL_ANGLE_keyed_mutex EGL_ANGLE_surface_orientation EGL_ANGLE_direct_composition EGL_NV_post_sub_buffer EGL_KHR_create_context EGL_EXT_device_query EGL_KHR_image EGL_KHR_image_base EGL_KHR_gl_texture_2D_image EGL_KHR_gl_texture_cubemap_image EGL_KHR_gl_renderbuffer_image EGL_KHR_get_all_proc_addresses EGL_KHR_stream EGL_KHR_stream_consumer_gltexture EGL_NV_stream_consumer_gltexture_yuv EGL_ANGLE_flexible_surface_compatibility EGL_ANGLE_stream_producer_d3d_texture EGL_ANGLE_create_context_webgl_compatibility EGL_CHROMIUM_create_context_bind_generates_resource EGL_CHROMIUM_sync_control EGL_EXT_pixel_format_float EGL_KHR_surfaceless_context EGL_ANGLE_display_texture_share_group EGL_ANGLE_create_context_client_arrays EGL_ANGLE_program_cache_control EGL_ANGLE_robust_resource_initialization EGL_EXTENSIONS(nullptr): EGL_EXT_client_extensions EGL_EXT_platform_base EGL_EXT_platform_device EGL_ANGLE_platform_angle EGL_ANGLE_platform_angle_d3d EGL_ANGLE_device_creation EGL_ANGLE_device_creation_d3d11 EGL_ANGLE_experimental_present_path EGL_KHR_client_get_all_proc_addresses

     WebGL 1 Driver Renderer: Google Inc. -- ANGLE (Intel(R) HD Graphics 4000 Direct3D11 vs_5_0 ps_5_0)
     WebGL 1 Driver Version: OpenGL ES 2.0 (ANGLE 2.1.0.7edc21933ef1)
     WebGL 1 Driver Extensions: GL_ANGLE_client_arrays GL_ANGLE_depth_texture GL_ANGLE_framebuffer_blit GL_ANGLE_framebuffer_multisample GL_ANGLE_instanced_arrays GL_ANGLE_lossy_etc_decode GL_ANGLE_pack_reverse_row_order GL_ANGLE_program_cache_control GL_ANGLE_request_extension GL_ANGLE_robust_client_memory GL_ANGLE_texture_compression_dxt3 GL_ANGLE_texture_compression_dxt5 GL_ANGLE_texture_usage GL_ANGLE_translated_shader_source GL_CHROMIUM_bind_generates_resource GL_CHROMIUM_bind_uniform_location GL_CHROMIUM_color_buffer_float_rgb GL_CHROMIUM_color_buffer_float_rgba GL_CHROMIUM_copy_compressed_texture GL_CHROMIUM_copy_texture GL_CHROMIUM_sync_query GL_EXT_blend_minmax GL_EXT_color_buffer_half_float GL_EXT_debug_marker GL_EXT_discard_framebuffer GL_EXT_disjoint_timer_query GL_EXT_draw_buffers GL_EXT_frag_depth GL_EXT_map_buffer_range GL_EXT_occlusion_query_boolean GL_EXT_read_format_bgra GL_EXT_robustness GL_EXT_sRGB GL_EXT_shader_texture_lod GL_EXT_texture_compression_dxt1 GL_EXT_texture_compression_s3tc_srgb GL_EXT_texture_filter_anisotropic GL_EXT_texture_format_BGRA8888 GL_EXT_texture_rg GL_EXT_texture_storage GL_EXT_unpack_subimage GL_KHR_debug GL_NV_EGL_stream_consumer_external GL_NV_fence GL_NV_pack_subimage GL_NV_pixel_buffer_object GL_OES_EGL_image GL_OES_EGL_image_external GL_OES_compressed_ETC1_RGB8_texture GL_OES_depth32 GL_OES_element_index_uint GL_OES_get_program_binary GL_OES_mapbuffer GL_OES_packed_depth_stencil GL_OES_rgb8_rgba8 GL_OES_standard_derivatives GL_OES_surfaceless_context GL_OES_texture_float GL_OES_texture_float_linear GL_OES_texture_half_float GL_OES_texture_half_float_linear GL_OES_texture_npot GL_OES_vertex_array_object
     WebGL 1 Extensions: ANGLE_instanced_arrays EXT_blend_minmax EXT_color_buffer_half_float EXT_frag_depth EXT_sRGB EXT_shader_texture_lod EXT_texture_filter_anisotropic EXT_disjoint_timer_query OES_element_index_uint OES_standard_derivatives OES_texture_float OES_texture_float_linear OES_texture_half_float OES_texture_half_float_linear OES_vertex_array_object WEBGL_color_buffer_float WEBGL_compressed_texture_s3tc WEBGL_compressed_texture_s3tc_srgb WEBGL_debug_renderer_info WEBGL_debug_shaders WEBGL_depth_texture WEBGL_draw_buffers WEBGL_lose_context
     WebGL 2 Driver WSI Info: EGL_VENDOR: Google Inc. (adapter LUID: 000000000001a495)

EGL_VERSION: 1.4 (ANGLE 2.1.0.7edc21933ef1) EGL_EXTENSIONS: EGL_EXT_create_context_robustness EGL_ANGLE_d3d_share_handle_client_buffer EGL_ANGLE_d3d_texture_client_buffer EGL_ANGLE_surface_d3d_texture_2d_share_handle EGL_ANGLE_query_surface_pointer EGL_ANGLE_window_fixed_size EGL_ANGLE_keyed_mutex EGL_ANGLE_surface_orientation EGL_ANGLE_direct_composition EGL_NV_post_sub_buffer EGL_KHR_create_context EGL_EXT_device_query EGL_KHR_image EGL_KHR_image_base EGL_KHR_gl_texture_2D_image EGL_KHR_gl_texture_cubemap_image EGL_KHR_gl_renderbuffer_image EGL_KHR_get_all_proc_addresses EGL_KHR_stream EGL_KHR_stream_consumer_gltexture EGL_NV_stream_consumer_gltexture_yuv EGL_ANGLE_flexible_surface_compatibility EGL_ANGLE_stream_producer_d3d_texture EGL_ANGLE_create_context_webgl_compatibility EGL_CHROMIUM_create_context_bind_generates_resource EGL_CHROMIUM_sync_control EGL_EXT_pixel_format_float EGL_KHR_surfaceless_context EGL_ANGLE_display_texture_share_group EGL_ANGLE_create_context_client_arrays EGL_ANGLE_program_cache_control EGL_ANGLE_robust_resource_initialization EGL_EXTENSIONS(nullptr): EGL_EXT_client_extensions EGL_EXT_platform_base EGL_EXT_platform_device EGL_ANGLE_platform_angle EGL_ANGLE_platform_angle_d3d EGL_ANGLE_device_creation EGL_ANGLE_device_creation_d3d11 EGL_ANGLE_experimental_present_path EGL_KHR_client_get_all_proc_addresses

     WebGL 2 Driver Renderer: Google Inc. -- ANGLE (Intel(R) HD Graphics 4000 Direct3D11 vs_5_0 ps_5_0)
     WebGL 2 Driver Version: OpenGL ES 3.0 (ANGLE 2.1.0.7edc21933ef1)
     WebGL 2 Driver Extensions: GL_ANGLE_client_arrays GL_ANGLE_depth_texture GL_ANGLE_framebuffer_blit GL_ANGLE_framebuffer_multisample GL_ANGLE_instanced_arrays GL_ANGLE_lossy_etc_decode GL_ANGLE_multiview GL_ANGLE_pack_reverse_row_order GL_ANGLE_program_cache_control GL_ANGLE_request_extension GL_ANGLE_robust_client_memory GL_ANGLE_texture_compression_dxt3 GL_ANGLE_texture_compression_dxt5 GL_ANGLE_texture_usage GL_ANGLE_translated_shader_source GL_CHROMIUM_bind_generates_resource GL_CHROMIUM_bind_uniform_location GL_CHROMIUM_color_buffer_float_rgb GL_CHROMIUM_color_buffer_float_rgba GL_CHROMIUM_copy_compressed_texture GL_CHROMIUM_copy_texture GL_CHROMIUM_sync_query GL_EXT_blend_minmax GL_EXT_color_buffer_float GL_EXT_color_buffer_half_float GL_EXT_debug_marker GL_EXT_discard_framebuffer GL_EXT_disjoint_timer_query GL_EXT_draw_buffers GL_EXT_frag_depth GL_EXT_map_buffer_range GL_EXT_occlusion_query_boolean GL_EXT_read_format_bgra GL_EXT_robustness GL_EXT_sRGB GL_EXT_shader_texture_lod GL_EXT_texture_compression_dxt1 GL_EXT_texture_compression_s3tc_srgb GL_EXT_texture_filter_anisotropic GL_EXT_texture_format_BGRA8888 GL_EXT_texture_norm16 GL_EXT_texture_rg GL_EXT_texture_storage GL_EXT_unpack_subimage GL_KHR_debug GL_NV_EGL_stream_consumer_external GL_NV_fence GL_NV_pack_subimage GL_NV_pixel_buffer_object GL_OES_EGL_image GL_OES_EGL_image_external GL_OES_EGL_image_external_essl3 GL_OES_compressed_ETC1_RGB8_texture GL_OES_depth32 GL_OES_element_index_uint GL_OES_get_program_binary GL_OES_mapbuffer GL_OES_packed_depth_stencil GL_OES_rgb8_rgba8 GL_OES_standard_derivatives GL_OES_surfaceless_context GL_OES_texture_float GL_OES_texture_float_linear GL_OES_texture_half_float GL_OES_texture_half_float_linear GL_OES_texture_npot GL_OES_vertex_array_object
     WebGL 2 Extensions: EXT_color_buffer_float EXT_texture_filter_anisotropic EXT_disjoint_timer_query OES_texture_float_linear WEBGL_compressed_texture_s3tc WEBGL_compressed_texture_s3tc_srgb WEBGL_debug_renderer_info WEBGL_debug_shaders WEBGL_lose_context
     Off Main Thread Painting Enabled: true
     DirectWrite: true (10.0.17134.1)

     GPU #1
     Active: Yes
     Description: Intel(R) HD Graphics 4000
     Vendor ID: 0x8086
     Device ID: 0x0166
     Driver Version: 10.18.10.4358
     Driver Date: 12-21-2015
     Drivers: igdumdim64 igd10iumd64 igd10iumd64 igdumdim32 igd10iumd32 igd10iumd32
     Subsys ID: 501817aa
     RAM: Unknown

   GPU #2

     Diagnostics
     ClearType Parameters: Gamma: 2.2 Pixel Structure: RGB ClearType Level: 50 Enhanced Contrast: 400
     AzureCanvasAccelerated: 0
     AzureCanvasBackend: skia
     AzureContentBackend: skia
     AzureFallbackCanvasBackend: cairo
     ClearType Parameters: Gamma: 2.2 Pixel Structure: RGB ClearType Level: 50 Enhanced Contrast: 400

     Decision Log
     HW_COMPOSITING: disabled by user: Disabled by pref
     D3D11_COMPOSITING: unavailable by default: Hardware compositing is disabled
     DIRECT2D: una

Please try this: Exit Thunderbird and other programs. then update your network adapter drivers. Some Windows info at link: https://support.microsoft.com/en-gb/help/4028443/windows-10-update-drivers

The problem was finally solved but I am not quite sure if this is how the problem was solved. What I did was I log in to Safe Mode of Windows 10, then I ran DISM and sfc /scannow as instructed in Microsoft troubleshooting webpage, which repaired some system files, started some services such as notify server. Then I rebooted to normal login. I started Thunderbird but still the same problem persisted but when I changed the smtp port to 465 from default 587, this time a security exception window pop up with a button to confirm security exception. I clicked that security exception button and then smtp started working and now I can send email. I suspect the Windows 10 was blocking smtp at port 587 for some certificate issue but when I changed port to 465 and 2525 few days ago, the security exception window never popped up and the outgoing email did not worked. Today, miraculously security exception window popped up and it happily put port 465 into security exception and I was able to send outgoing email. So, I think the problem was in Windows 10 itself and I think it was related security. I don't understand why all of a sudden smtp port 587 was blocked by windows 10 or why smtp port 587 was removed or not offered to be put in security exception.

The security exception that I mentioned in my earlier post was not from Windows but it was from Thunderbird itself. It seems Thunderbird is very critical about security certificates of mail servers and it blocks one that does not meet its criteria. I guess that's what was happening to my email server's smtp security certificate. After some research, I found the certificate management in Thunderbird is under Menu (three horizontal dashes on far right top corner) -> Options -> Advanced Tab. There's a button "Manage Certificates" over there. When clicked "Certificate Manager" window popped up and smtp server was under 'Servers' tab. One should keep the certificate under security exception with permanent lifetime so as to keep the outgoing email working.

One should keep the certificate under security exception with permanent lifetime

One shouldn't need to create a security exception in the first place. If everything is good with your email provider's server cert, you wouldn't get an exception prompt. So something is wrong with the cert at best, or you reach to a malicious server in the worst case. When creating an exception you must do this evaluation and decide what to do. Better yet is to have the underlying problem fixed.

Thanks Christ1 for your valuable information. I guess my email server imap and smtp certificates are not up to date or is not at par with the requirement of Thunderbird application. However, the email server itself is genuine site not malicious. Due to issue with server certificate, the Thunderbird application was blocking outgoing email which was really annoying when lots of work has to be done through email. I guess only few people will encounter this sort of problem but after much research and hit and trial, I was able to sort out the problem. What I did was, I deleted the smtp certificates listed in Options -> Advanced Tab-> "Manage Certificates" button ->"Certificate Manager/ Server Tab. Then close the Thunderbird application and restart the PC itself. After restarting, open the Thunderbird application and now when it tried to connect the smtp server (when trying to send outgoing email) , it asked for exception which I gladly provided and it start to send email again.

The test results on the mail server incoming give it a fail... there are just so many things wrong it is not funny.

Including the use of Symantec server certificates. They will cause it to stop working very very soon. It is a race between their expiry on the 25th and their official distrusting. Without serious updating that mails server days are about over. https://www.htbridge.com/ssl/?id=kVnKcRee

Problems identified are

The server's Diffie-Hellman parameter is too small. The server supports protocols that have known weaknesses and are considered unsafe. The server supports cipher suites that are not approved by PCI DSS requirements, NIST guidelines and HIPAA guidance. The server's certificate is untrusted. The server has TLS 1.0 enabled. Since the 30th of June 2018 it is non-compliant with PCI DSS 3.2.1. The server is vulnerable to POODLE over SSL. CA of your SSL certificate is distrusted by Google Chrome and Mozilla Firefox. (and I will add and Mozilla Thunderbird)

A test on smtp.ntc.net.np:587 tells me that the server simply has no certificate, so certificate error should not exist. It is not capable of encrypted communication. All SMTP should be set up to use no connection security. Plain text I think the term account settings uses.

https://www.htbridge.com/ssl/?id=udGaEODI

I suggest you stop trying to make Thunderbird work with this server and fix the serious issues it has. It is certainly not suitable to undertake any sort of business with where data security is even slightly important.

Thanks Matt for pointing out so many vulnerability of the email server. I shall point this out to the concerned admin of the email server. Thanks once again.