Open your error - console (Ctrl+Shift+J) Clear it. Open sent Tell us what error you get in the console.

Is it a POP or IMAP-account?

Cleared the console. tRied to open sent. No error message now in console. It is an IMAP account

Keeps saying the current operation on Sent did not succeeed. Eventually it shows the sent messages but situation getting worse

This error in console box now:

2015-09-29 18:17:24 autosyncActivities ERROR onDownloadCompleted: TypeError: this._syncInfoPerFolder[folder.URI] is undefined

Have you many mails in your inbox, sent, trash and junk folders? Have you regulary compacted those folders? Is sent local or on server? Sent has a way of being overlooked and is never cleaned or compacted.

I've asked about this.

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 also add the result of these troubleshooting steps to your post

• Does Thunderbird work in TB Safe mode (see Thunderbird Safe Mode)?
• Do you use anti-virus and firewall software? What is the version?
• Who is the email provider?
• Who is the internet provider?
• What is the exact error message?
• What steps did you take right before this happened?

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

also see https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems

Posted from help:

 Application Basics

   Name: Thunderbird
   Version: 38.2.0
   User Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20150813074416
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (imap) imap-mail.outlook.com:993, SSL, passwordCleartext
     OUTGOING: smtp-mail.outlook.com:587, alwaysSTARTTLS, passwordCleartext, true

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

 Crash Reports

 Extensions
   Lightning, 4.0.2.1, false, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     browser.cache.disk.capacity: 358400
     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: 38.2.0
     font.name.monospace.el: 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.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.x-cyrillic: Cambria
     font.name.serif.x-unicode: Cambria
     font.name.serif.x-western: Cambria
     font.size.fixed.el: 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.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: 6a370e74-d471-4b4a-8536-08cfa8550a0
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1443514132
     places.history.expiration.transient_current_max_pages: 104858
     plugin.importedState: true

 Graphics

     Adapter Description: Intel(R) HD Graphics 4000
     Vendor ID: 0x8086
     Device ID: 0x0166
     Adapter RAM: Unknown
     Adapter Drivers: igdumdim64 igd10iumd64 igd10iumd64 igdumdim32 igd10iumd32 igd10iumd32
     Driver Version: 10.18.10.3958
     Driver Date: 9-30-2014
     Adapter Description (GPU #2): NVIDIA GeForce GT 630M
     Vendor ID (GPU #2): 0x10de
     Device ID (GPU #2): 0x0de9
     Adapter RAM (GPU #2): 1024
     Adapter Drivers (GPU #2): nvd3dumx,nvwgf2umx,nvwgf2umx nvd3dum,nvwgf2um,nvwgf2um
     Driver Version (GPU #2): 9.18.13.2702
     Driver Date (GPU #2): 8-29-2013
     Direct2D Enabled: false
     DirectWrite Enabled: false (6.3.9600.17999)
     ClearType Parameters: ClearType parameters not found
     WebGL Renderer: false
     GPU Accelerated Windows: 0

     AzureCanvasBackend: skia
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: cairo

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.10.8
     4.10.8

     NSS
     3.19.2 Basic ECC
     3.19.2 Basic ECC

     NSS Util
     3.19.2
     3.19.2

     NSS SSL
     3.19.2 Basic ECC
     3.19.2 Basic ECC

     NSS S/MIME
     3.19.2 Basic ECC
     3.19.2 Basic ECC

Windows 8.1 - HOtmail account. AVG full version. The error I am receiving is that thunderbird is downloading hundreds of headers from the sent box.

I am a pensioner and not technical so you will have to reply in layman's terms