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?

See the info below. Also, I use MacAfee Security Center for my anti-virus and firewall, and the Version is 16.0.1.

I started Thunderbird in safe mode but no success in sending emails. Although I did send one last night to a friend and they received it.

 Application Basics

   Name: Thunderbird
   Version: 52.3.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; 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
   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account7:
     INCOMING: account7, , (pop3) pop.centurylink.net:995, SSL, passwordCleartext
     OUTGOING: , smtp.centurylink.net:587, alwaysSTARTTLS, passwordCleartext, true

   account8:
     INCOMING: account8, , (pop3) pop3.live.com:995, SSL, passwordCleartext
     OUTGOING: , smtp.live.com:587, alwaysSTARTTLS, passwordCleartext, true

 Crash Reports
   https://crash-stats.mozilla.com/report/index/bp-892e3c39-ee5c-45cb-b60d-fcd172160311 (3/11/2016)
   https://crash-stats.mozilla.com/report/index/bp-72555b27-1c6b-4d68-b029-a5a142141123 (11/23/2014)
   https://crash-stats.mozilla.com/report/index/bp-9e3ba79e-4fd6-4147-a946-03a322140923 (9/23/2014)
   https://crash-stats.mozilla.com/report/index/bp-49e4ec47-c9bc-401c-84c6-556272140425 (4/25/2014)
   https://crash-stats.mozilla.com/report/index/bp-b9572ec7-b627-4f9f-8c62-d18db2140422 (4/22/2014)

 Extensions
   Lightning, 5.4.3, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   McAfee Anti-Spam Thunderbird Extension, 3.0, true, [email protected]

 Important Modified Preferences

   Name: Value

     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
     dom.max_chrome_script_run_time: 0
     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.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: 14
     gfx.blacklist.suggested-driver-version: 257.21
     gfx.direct3d.last_used_feature_level_idx: 1
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: 2ebfb566-6231-449a-b637-4c73e51a1d6
     media.gmp.storage.version.observed: 1
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1504671473
     places.history.expiration.transient_current_max_pages: 122334
     plugin.disable_full_page_plugin_for_types: application/pdf
     plugin.importedState: true
     security.sandbox.content.tempDirSuffix: {95173d02-9d2b-430e-bf55-c29979e7ea49}

 Graphics

     GPU #1
     Description: NVIDIA GeForce 8600 GT
     Vendor ID: 0x10de
     Device ID: 0x0402
     RAM: 256
     Drivers: nvd3dum nvwgf2um,nvwgf2um
     Driver Version: 21.21.13.4201
     Driver Date: 11-14-2016

     Features
     Direct2D: false
     DirectWrite: true (10.0.15063.413)
     ClearType Parameters: Gamma: 2.2 Pixel Structure: RGB ClearType Level: 100 Enhanced Contrast: 400
     WebGL Renderer: Google Inc. -- ANGLE (NVIDIA GeForce 8600 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

There have been other reports here complaining about a similar issue with Centurylink. https://support.mozilla.org/en-US/questions/1174317

Wait for three days until you get the bounce error message and post the error code. It seem Centurylink have screwed up their server configuration, so that messages from the Centurylink SMTP server are rejected by SMTP servers from other email providers.

It doesn't solve my issue but I understand the situation. In reading of others with this issue I got the same line from Centurylink....it must be your client cause our webmail works. The only difference is I'm not getting any error messages or bouncebacks. Nothing has shown up as undeliverable and if I check my Sent folder in TB there sits my email just like it has been sent. Guess I'll wait and see for a few days.

Ok so centurylink works sometimes and sometimes not. But here is a copy of an error message i just received upon trying to reply to an email.

"An error occurred while sending mail: The mail server sent an incorrect greeting: 4.3.4 allocated resources exceeded."

Any idea what this means?

You really should ask Centurylink about the error. It's from their server.