Please post your settings. 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 this:

• 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 text to clipboard to select all the text and copy it;
• Paste this in your reply to this forum.

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

Application Basics

   Name: Thunderbird
   Version: 52.5.0
   User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20171121193617
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account1:
     INCOMING: account1, , (imap) imap.googlemail.com:993, SSL, OAuth2
     OUTGOING: , smtp.googlemail.com:465, SSL, passwordCleartext, true

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

 Crash Reports

 Extensions
   Lightning, 5.4.5, 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: 52.5.0
     font.internaluseonly.changed: true
     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.direct3d.last_used_feature_level_idx: 0
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: 9e5d0c6a-7bd6-402c-a466-fb8624f0187
     mailnews.database.global.views.conversation.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: 1513184170
     places.history.expiration.transient_current_max_pages: 122334
     plugin.importedState: true
     print.printer_HP_Photosmart_Premium_C309g-m.print_bgcolor: false
     print.printer_HP_Photosmart_Premium_C309g-m.print_bgimages: false
     print.printer_HP_Photosmart_Premium_C309g-m.print_evenpages: true
     print.printer_HP_Photosmart_Premium_C309g-m.print_footercenter:
     print.printer_HP_Photosmart_Premium_C309g-m.print_footerleft: &PT
     print.printer_HP_Photosmart_Premium_C309g-m.print_footerright: &D
     print.printer_HP_Photosmart_Premium_C309g-m.print_headercenter:
     print.printer_HP_Photosmart_Premium_C309g-m.print_headerleft: &T
     print.printer_HP_Photosmart_Premium_C309g-m.print_headerright: &U
     print.printer_HP_Photosmart_Premium_C309g-m.print_in_color: true
     print.printer_HP_Photosmart_Premium_C309g-m.print_margin_bottom: 0.5
     print.printer_HP_Photosmart_Premium_C309g-m.print_margin_left: 0.5
     print.printer_HP_Photosmart_Premium_C309g-m.print_margin_right: 0.5
     print.printer_HP_Photosmart_Premium_C309g-m.print_margin_top: 0.5
     print.printer_HP_Photosmart_Premium_C309g-m.print_oddpages: true
     print.printer_HP_Photosmart_Premium_C309g-m.print_orientation: 0
     print.printer_HP_Photosmart_Premium_C309g-m.print_reversed: false
     print.printer_HP_Photosmart_Premium_C309g-m.print_scaling: 1.00
     print.printer_HP_Photosmart_Premium_C309g-m.print_shrink_to_fit: true
     security.sandbox.content.tempDirSuffix: {93e6ca89-bbe5-4022-920c-5558a2469648}

 Graphics

     GPU #1
     Description: Intel(R) HD Graphics 4000
     Vendor ID: 0x8086
     Device ID: 0x0166
     RAM: Unknown
     Drivers: igdumd64 igd10umd64 igd10umd64 igdumd32 igd10umd32 igd10umd32
     Driver Version: 8.15.10.2639
     Driver Date: 2-1-2012

     Features
     Direct2D: false
     DirectWrite: true (6.2.9200.22164)
     WebGL Renderer: Google Inc. -- ANGLE (Intel(R) HD Graphics 4000 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.

OUTGOING: , smtp.googlemail.com:465, SSL, passwordCleartext, true

I would recommend using OAuth2 for the SMTP server, as you have done for the incoming IMAP service.

The easiest way to set this up is to simply delete this account in Thunderbird and let it set it up again, allowing it to proceed with OAuth2 for both incoming (IMAP) and outgoing (SMTP). As this account uses IMAP, all your data is stored on gmail's server and you won't lose anything.

I suspect that the current SMTP setting using regular SSL will want you to supplement it with an application-specific password, or to enable two-factor authentication. Of all the permutations, OAuth2 is (IMHO) rather the easiest to use.

Thank you very much. I can now send emails...!