Application Basics

   Name: Thunderbird
   Version: 52.4.0
   User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0
   Profile Directory: Open Directory

             (Local drive)
   Application Build ID: 20171009154419
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account1:
     INCOMING: account1, , (imap) imap.gmail.com:993, SSL, OAuth2
     OUTGOING: , smtp.gmail.com:587, alwaysSTARTTLS, OAuth2, true

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

   account3:
     INCOMING: account3, , (imap) imap.gmail.com:993, SSL, OAuth2
     OUTGOING: , smtp.gmail.com:465, SSL, OAuth2, true

   account4:
     INCOMING: account4, , (imap) imap.gmail.com:993, SSL, OAuth2
     OUTGOING: , smtp.gmail.com:465, SSL, OAuth2, true

   account5:
     INCOMING: account5, , (im) prpl-gtalk, plain, 0

 Crash Reports

 Extensions
   Lightning, 5.4, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   Messaging Menu and Unity Launcher integration, 1.3.1, true, messagingmenu@mozilla.com
   Provider for Google Calendar, 3.3, true, {a62ef8ec-5fdc-40c2-873c-223b8a6925cc}
   Signature Switch, 1.6.16, true, {2ab1b709-ba03-4361-abf9-c50b964ff75d}

 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
     extensions.lastAppVersion: 52.4.0
     font.internaluseonly.changed: true
     mail.openMessageBehavior.version: 1
     mailnews.database.global.datastore.id: 86b90dd4-de1f-46ca-80e3-29fa31b924d
     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: 1509177816
     places.history.expiration.transient_current_max_pages: 122334
     plugin.importedState: true
     print.print_bgcolor: false
     print.print_bgimages: false
     print.print_duplex: 0
     print.print_evenpages: true
     print.print_in_color: true
     print.print_margin_bottom: 0.5
     print.print_margin_left: 0.5
     print.print_margin_right: 0.5
     print.print_margin_top: 0.5
     print.print_oddpages: true
     print.print_orientation: 0
     print.print_page_delay: 50
     print.print_paper_data: 0
     print.print_paper_height: 11.69
     print.print_paper_name: iso_a4
     print.print_paper_size_unit: 0
     print.print_paper_width: 8.27
     print.print_scaling: 1.00
     print.print_shrink_to_fit: true
     print.print_to_file: false
     print.print_unwriteable_margin_bottom: 56
     print.print_unwriteable_margin_left: 25
     print.print_unwriteable_margin_right: 25
     print.print_unwriteable_margin_top: 25

 Graphics

     GPU #1
     Description: Intel Open Source Technology Center -- Mesa DRI Intel(R) HD Graphics 520 (Skylake GT2)
     Vendor ID: Intel Open Source Technology Center
     Device ID: Mesa DRI Intel(R) HD Graphics 520 (Skylake GT2)
     Driver Version: 3.0 Mesa 11.2.0

     Features
     WebGL Renderer: Intel Open Source Technology Center -- Mesa DRI Intel(R) HD Graphics 520 (Skylake GT2)  -- 3.0 Mesa 11.2.0

     AzureCanvasBackend: skia
     AzureCanvasAccelerated: 0
     AzureFallbackCanvasBackend: none
     AzureContentBackend: skia
     CairoUseXRender: 0

 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 have TB 52.4 on Linux Mint 18 (Ubuntu 16.04). In my work network, every time I try to send a mail from my work (google apps account) or personal (gmail account), TB tries to connect to the smtp server and responds with a 'connection refused message' (see screenshot). This situation persists for a couple of minutes, and then somehow resolves itself, and the mail goes out. It happens every time, but only on my work network. At home and on say, phone hotspot, this does not happen. Pasting below trouble shooting info from TB: Application Basics Name: Thunderbird Version: 52.4.0 User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 Profile Directory: Open Directory (Local drive) Application Build ID: 20171009154419 Enabled Plugins: about:plugins Build Configuration: about:buildconfig Memory Use: about:memory Profiles: about:profiles Mail and News Accounts account1: INCOMING: account1, , (imap) imap.gmail.com:993, SSL, OAuth2 OUTGOING: , smtp.gmail.com:587, alwaysSTARTTLS, OAuth2, true account2: INCOMING: account2, , (none) Local Folders, plain, passwordCleartext account3: INCOMING: account3, , (imap) imap.gmail.com:993, SSL, OAuth2 OUTGOING: , smtp.gmail.com:465, SSL, OAuth2, true account4: INCOMING: account4, , (imap) imap.gmail.com:993, SSL, OAuth2 OUTGOING: , smtp.gmail.com:465, SSL, OAuth2, true account5: INCOMING: account5, , (im) prpl-gtalk, plain, 0 Crash Reports Extensions Lightning, 5.4, true, {e2fda1a4-762b-4020-b5ad-a41df1933103} Messaging Menu and Unity Launcher integration, 1.3.1, true, messagingmenu@mozilla.com Provider for Google Calendar, 3.3, true, {a62ef8ec-5fdc-40c2-873c-223b8a6925cc} Signature Switch, 1.6.16, true, {2ab1b709-ba03-4361-abf9-c50b964ff75d} 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 extensions.lastAppVersion: 52.4.0 font.internaluseonly.changed: true mail.openMessageBehavior.version: 1 mailnews.database.global.datastore.id: 86b90dd4-de1f-46ca-80e3-29fa31b924d 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: 1509177816 places.history.expiration.transient_current_max_pages: 122334 plugin.importedState: true print.print_bgcolor: false print.print_bgimages: false print.print_duplex: 0 print.print_evenpages: true print.print_in_color: true print.print_margin_bottom: 0.5 print.print_margin_left: 0.5 print.print_margin_right: 0.5 print.print_margin_top: 0.5 print.print_oddpages: true print.print_orientation: 0 print.print_page_delay: 50 print.print_paper_data: 0 print.print_paper_height: 11.69 print.print_paper_name: iso_a4 print.print_paper_size_unit: 0 print.print_paper_width: 8.27 print.print_scaling: 1.00 print.print_shrink_to_fit: true print.print_to_file: false print.print_unwriteable_margin_bottom: 56 print.print_unwriteable_margin_left: 25 print.print_unwriteable_margin_right: 25 print.print_unwriteable_margin_top: 25 Graphics GPU #1 Description: Intel Open Source Technology Center -- Mesa DRI Intel(R) HD Graphics 520 (Skylake GT2) Vendor ID: Intel Open Source Technology Center Device ID: Mesa DRI Intel(R) HD Graphics 520 (Skylake GT2) Driver Version: 3.0 Mesa 11.2.0 Features WebGL Renderer: Intel Open Source Technology Center -- Mesa DRI Intel(R) HD Graphics 520 (Skylake GT2) -- 3.0 Mesa 11.2.0 AzureCanvasBackend: skia AzureCanvasAccelerated: 0 AzureFallbackCanvasBackend: none AzureContentBackend: skia CairoUseXRender: 0 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