Care to post the error you get?

I have used Thunderbird for months. However, in the last couple of weeks, I constantly get smtp errors. I used to be able to close the error & resend, however, that is not working anymore. My password has not changed. Sometimes, I don't even get an error, it just says Sending. This is also happening to another user. We both are syncing with yahoo accounts. Nothing has been changed on our end, updates are current. Why can't I get my emails to send?

Can you post your Troubleshooting Information? Help (Alt-H) - Troubleshooting Information

Sure, here is the Troubleshooting Information

Application Basics

   Name: Thunderbird
   Version: 31.4.0
   User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20150109111741
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (none) Local Folders, plain, passwordCleartext

   account6:
     INCOMING: account6, , (imap) imap.mail.yahoo.com:993, SSL, passwordCleartext
     OUTGOING: smtp.mail.yahoo.com:465, SSL, passwordCleartext, true

 Crash Reports
   http://crash-stats.mozilla.com/report/index/bp-bb6dd6a6-c558-4d84-b159-5651e2150129 (01/29/2015)

 Extensions
   Enigmail, 1.7.2, true, {847b3a00-7ab1-11d4-8f02-006008948af5}
   Lightning, 3.3.3, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     accessibility.typeaheadfind.flashBar: 0
     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: 31.4.0
     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
     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: 18814dea-9dca-45b5-be17-1dfc2922ad8
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     network.cookie.prefsMigrated: true
     places.database.lastMaintenance: 1424096202
     places.history.expiration.transient_current_max_pages: 104858
     plugin.importedState: true
     print.printer_HP_LaserJet_Professional_P1102w.print_bgcolor: false
     print.printer_HP_LaserJet_Professional_P1102w.print_bgimages: false
     print.printer_HP_LaserJet_Professional_P1102w.print_colorspace:
     print.printer_HP_LaserJet_Professional_P1102w.print_command:
     print.printer_HP_LaserJet_Professional_P1102w.print_downloadfonts: false
     print.printer_HP_LaserJet_Professional_P1102w.print_duplex: 47
     print.printer_HP_LaserJet_Professional_P1102w.print_edge_bottom: 0
     print.printer_HP_LaserJet_Professional_P1102w.print_edge_left: 0
     print.printer_HP_LaserJet_Professional_P1102w.print_edge_right: 0
     print.printer_HP_LaserJet_Professional_P1102w.print_edge_top: 0
     print.printer_HP_LaserJet_Professional_P1102w.print_evenpages: true
     print.printer_HP_LaserJet_Professional_P1102w.print_footercenter:
     print.printer_HP_LaserJet_Professional_P1102w.print_footerleft: &PT
     print.printer_HP_LaserJet_Professional_P1102w.print_footerright: &D
     print.printer_HP_LaserJet_Professional_P1102w.print_headercenter:
     print.printer_HP_LaserJet_Professional_P1102w.print_headerleft: &T
     print.printer_HP_LaserJet_Professional_P1102w.print_headerright: &U
     print.printer_HP_LaserJet_Professional_P1102w.print_in_color: true
     print.printer_HP_LaserJet_Professional_P1102w.print_margin_bottom: 0.5
     print.printer_HP_LaserJet_Professional_P1102w.print_margin_left: 0.5
     print.printer_HP_LaserJet_Professional_P1102w.print_margin_right: 0.5
     print.printer_HP_LaserJet_Professional_P1102w.print_margin_top: 0.5
     print.printer_HP_LaserJet_Professional_P1102w.print_oddpages: true
     print.printer_HP_LaserJet_Professional_P1102w.print_orientation: 0
     print.printer_HP_LaserJet_Professional_P1102w.print_page_delay: 50
     print.printer_HP_LaserJet_Professional_P1102w.print_paper_data: 1
     print.printer_HP_LaserJet_Professional_P1102w.print_paper_height: 11.00
     print.printer_HP_LaserJet_Professional_P1102w.print_paper_name:
     print.printer_HP_LaserJet_Professional_P1102w.print_paper_size_type: 0
     print.printer_HP_LaserJet_Professional_P1102w.print_paper_size_unit: 0
     print.printer_HP_LaserJet_Professional_P1102w.print_paper_width: 8.50
     print.printer_HP_LaserJet_Professional_P1102w.print_plex_name:
     print.printer_HP_LaserJet_Professional_P1102w.print_resolution: -1
     print.printer_HP_LaserJet_Professional_P1102w.print_resolution_name:
     print.printer_HP_LaserJet_Professional_P1102w.print_reversed: false
     print.printer_HP_LaserJet_Professional_P1102w.print_scaling: 1.00
     print.printer_HP_LaserJet_Professional_P1102w.print_shrink_to_fit: true
     print.printer_HP_LaserJet_Professional_P1102w.print_to_file: false
     print.printer_HP_LaserJet_Professional_P1102w.print_unwriteable_margin_bottom: 0
     print.printer_HP_LaserJet_Professional_P1102w.print_unwriteable_margin_left: 0
     print.printer_HP_LaserJet_Professional_P1102w.print_unwriteable_margin_right: 0
     print.printer_HP_LaserJet_Professional_P1102w.print_unwriteable_margin_top: 0

 Graphics

     Adapter Description: Intel(R) HD Graphics
     Vendor ID: 0x8086
     Device ID: 0x0102
     Adapter RAM: Unknown
     Adapter Drivers: igdumd64 igd10umd64 igd10umd64 igdumd32 igd10umd32 igd10umd32
     Driver Version: 9.17.10.2867
     Driver Date: 9-26-2012
     Direct2D Enabled: true
     DirectWrite Enabled: true (6.2.9200.16571)
     ClearType Parameters: Gamma: 1800 Pixel Structure: R ClearType Level: 50 Enhanced Contrast: 50
     WebGL Renderer: false
     GPU Accelerated Windows: 2/2 Direct3D 10

     AzureCanvasBackend: direct2d
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: direct2d

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.10.6
     4.10.6

     NSS
     3.16.2.3 Basic ECC
     3.16.2.3 Basic ECC

     NSS Util
     3.16.2.3
     3.16.2.3

     NSS SSL
     3.16.2.3 Basic ECC
     3.16.2.3 Basic ECC

     NSS S/MIME
     3.16.2.3 Basic ECC
     3.16.2.3 Basic ECC

Your server settings look good. For me Yahoo isn't the world's most reliable email service, so this may be a temporary Yahoo glitch. Have you tried to restart Thunderbird?

I have tried to restart several times. It will send messages a few times & then go right back to the errors. It's getting very frustrating. I use Yahoo for my personal email & have never had issues. I have been using Yahoo/Thunderbird for over a year now at work & this just cropped up.