This makes me so angry. If they don't know what they are doing then they should say so and not attempt to fiddle with it. Or get competent staff to handle these situations properly.

Is hard to know exactly what they have done. There is a chance your messages are still stored somewhere and will reappear when the connection is restored.

Uninstalling and reinstalling Thunderbird is unlikely to help, since it is the settings that are amiss and these are not affected by the installation process.

It would be useful to know 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 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 post back with this information and any other questions you might have.

Application Basics

   Name: Thunderbird
   Version: 52.3.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20170815040357
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account3:
     INCOMING: account3, , (none) Local Folders, plain, passwordCleartext

 Crash Reports
   https://crash-stats.mozilla.com/report/index/bp-102ddfc4-0057-44e3-8433-f71b02140315 (3/15/2014)
   https://crash-stats.mozilla.com/report/index/bp-90049bee-194d-4df9-bb46-3e6fa2131208 (12/8/2013)
   https://crash-stats.mozilla.com/report/index/bp-b07cb84f-83a1-4458-8105-56ca32121102 (11/2/2012)
   https://crash-stats.mozilla.com/report/index/bp-9b65e4bd-8792-41b7-8185-a0abf2120426 (4/26/2012)
   https://crash-stats.mozilla.com/report/index/bp-a3991b4c-f4da-420a-8d5c-d20112120104 (1/4/2012)

 Extensions

 Important Modified Preferences

   Name: Value

     accessibility.lastLoadDate: 1506179486
     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: 52.3.0
     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
     gfx.crash-guard.glcontext.appVersion: 45.1.1
     gfx.crash-guard.glcontext.deviceID: 0x0421
     gfx.crash-guard.glcontext.driverVersion: 9.18.13.4195
     gfx.crash-guard.glcontext.gfx.driver-init.direct3d11-angle: true
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle: true
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-force-d3d11: false
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-force-warp: false
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-try-d3d11: true
     gfx.crash-guard.status.glcontext: 2
     gfx.direct3d.last_used_feature_level_idx: 1
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: 7e5fd4b4-b0b5-4dc6-99e3-7e03bec1117
     media.gmp.storage.version.observed: 1
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1506024778
     places.history.expiration.transient_current_max_pages: 122334
     plugin.importedState: true
     print.printer_Brother_MFC-J450DW_Printer.print_bgcolor: false
     print.printer_Brother_MFC-J450DW_Printer.print_bgimages: false
     print.printer_Brother_MFC-J450DW_Printer.print_colorspace:
     print.printer_Brother_MFC-J450DW_Printer.print_command:
     print.printer_Brother_MFC-J450DW_Printer.print_downloadfonts: false
     print.printer_Brother_MFC-J450DW_Printer.print_duplex: -437918235
     print.printer_Brother_MFC-J450DW_Printer.print_edge_bottom: 0
     print.printer_Brother_MFC-J450DW_Printer.print_edge_left: 0
     print.printer_Brother_MFC-J450DW_Printer.print_edge_right: 0
     print.printer_Brother_MFC-J450DW_Printer.print_edge_top: 0
     print.printer_Brother_MFC-J450DW_Printer.print_evenpages: true
     print.printer_Brother_MFC-J450DW_Printer.print_footercenter:
     print.printer_Brother_MFC-J450DW_Printer.print_footerleft: &PT
     print.printer_Brother_MFC-J450DW_Printer.print_footerright: &D
     print.printer_Brother_MFC-J450DW_Printer.print_headercenter:
     print.printer_Brother_MFC-J450DW_Printer.print_headerleft: &T
     print.printer_Brother_MFC-J450DW_Printer.print_headerright: &U
     print.printer_Brother_MFC-J450DW_Printer.print_in_color: true
     print.printer_Brother_MFC-J450DW_Printer.print_margin_bottom: 0.5
     print.printer_Brother_MFC-J450DW_Printer.print_margin_left: 0.5
     print.printer_Brother_MFC-J450DW_Printer.print_margin_right: 0.5
     print.printer_Brother_MFC-J450DW_Printer.print_margin_top: 0.5
     print.printer_Brother_MFC-J450DW_Printer.print_oddpages: true
     print.printer_Brother_MFC-J450DW_Printer.print_orientation: 0
     print.printer_Brother_MFC-J450DW_Printer.print_page_delay: 50
     print.printer_Brother_MFC-J450DW_Printer.print_paper_data: 1
     print.printer_Brother_MFC-J450DW_Printer.print_paper_height: 11.00
     print.printer_Brother_MFC-J450DW_Printer.print_paper_name:
     print.printer_Brother_MFC-J450DW_Printer.print_paper_size_type: 0
     print.printer_Brother_MFC-J450DW_Printer.print_paper_size_unit: 0
     print.printer_Brother_MFC-J450DW_Printer.print_paper_width: 8.50
     print.printer_Brother_MFC-J450DW_Printer.print_plex_name:
     print.printer_Brother_MFC-J450DW_Printer.print_resolution: -437918235
     print.printer_Brother_MFC-J450DW_Printer.print_resolution_name:
     print.printer_Brother_MFC-J450DW_Printer.print_reversed: false
     print.printer_Brother_MFC-J450DW_Printer.print_scaling: 0.80
     print.printer_Brother_MFC-J450DW_Printer.print_shrink_to_fit: false
     print.printer_Brother_MFC-J450DW_Printer.print_to_file: false
     print.printer_Brother_MFC-J450DW_Printer.print_unwriteable_margin_bottom: 0
     print.printer_Brother_MFC-J450DW_Printer.print_unwriteable_margin_left: 0
     print.printer_Brother_MFC-J450DW_Printer.print_unwriteable_margin_right: 0
     print.printer_Brother_MFC-J450DW_Printer.print_unwriteable_margin_top: 0
     security.default_personal_cert: Select Automatically
     security.sandbox.content.tempDirSuffix: {650158ef-e390-4a1f-8e60-2c9e2d63eaac}

 Graphics

     GPU #1
     Description: NVIDIA GeForce 8500 GT
     Vendor ID: 0x10de
     Device ID: 0x0421
     RAM: 256
     Drivers: nvd3dumx,nvwgf2umx,nvwgf2umx nvd3dum,nvwgf2um,nvwgf2um
     Driver Version: 21.21.13.4201
     Driver Date: 11-14-2016

     Features
     Direct2D: false
     DirectWrite: true (10.0.15063.413)
     WebGL Renderer: Google Inc. -- ANGLE (NVIDIA GeForce 8500 GT 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

3.28.5