Yahoo gave you this link https://help.yahoo.com/kb/SLN7253.html?impressions=true to explain their reasons. IT is mostly rubbish, but basically they have made radical changes to what they will accept in the way of mail.

Please Copy and paste the troubleshooting information from Thunderbird help menu into a massage here so we can check your settings to see perhaps what settings you have wrong.

I do not find a section in TB Help which gives my settings. Frank

Is this what you wanted?:

 Application Basics

   Name: Thunderbird
   Version: 24.6.0
   User Agent: Mozilla/5.0 (Windows NT 5.1; rv:24.0) Gecko/20100101 Thunderbird/24.6.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20140610001341
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Crash Reports: about:crashes
   Memory Use: about:memory

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

   account2:
     INCOMING: account2, , (imap) imap.cs.com:143, plain, passwordCleartext
     OUTGOING: smtp-server.cfl.rr.com:25, plain, none, true

   account3:
     INCOMING: account3, , (pop3) pop-server.cfl.rr.com:110, plain, passwordCleartext
     OUTGOING: smtp-server.cfl.rr.com:25, plain, none, true

   account5:
     INCOMING: account5, , (imap) imap.aol.com:993, SSL, passwordCleartext
     OUTGOING: smtp.aol.com:587, alwaysSTARTTLS, passwordCleartext, true

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

   account7:
     INCOMING: account7, , (imap) imap.aol.com:993, SSL, passwordCleartext
     OUTGOING: smtp.aol.com:587, alwaysSTARTTLS, passwordCleartext, true

 Extensions

 Important Modified Preferences

   Name: Value

     accessibility.typeaheadfind.flashBar: 0
     browser.cache.disk.capacity: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     browser.cache.disk.smart_size_cached_value: 358400
     extensions.lastAppVersion: 24.6.0
     font.internaluseonly.changed: false
     font.name.sans-serif.x-western: Arial Black
     mail.openMessageBehavior.version: 1
     mailnews.database.global.datastore.id: bed986a0-ba91-4727-8298-0e71c822981
     network.cookie.prefsMigrated: true
     places.database.lastMaintenance: 1402666221
     places.history.expiration.transient_current_max_pages: 25992
     places.history.expiration.transient_optimal_database_size: 41585868
     plugin.importedState: true
     plugin.state.java: 0
     plugin.state.npcig: 0
     plugin.state.npdeployjava: 0
     plugin.state.npdrmv: 0
     plugin.state.npdsplay: 0
     plugin.state.npgeplugin: 0
     plugin.state.nppdf: 0
     plugin.state.npqtplugin: 0
     plugin.state.npwmsdrm: 0
     plugin.state.npwpf: 0
     print.print_printer: EPSON Stylus Photo R1800
     print.printer_EPSON_Stylus_Photo_R1800.print_bgcolor: false
     print.printer_EPSON_Stylus_Photo_R1800.print_bgimages: false
     print.printer_EPSON_Stylus_Photo_R1800.print_command:
     print.printer_EPSON_Stylus_Photo_R1800.print_downloadfonts: false
     print.printer_EPSON_Stylus_Photo_R1800.print_edge_bottom: 0
     print.printer_EPSON_Stylus_Photo_R1800.print_edge_left: 0
     print.printer_EPSON_Stylus_Photo_R1800.print_edge_right: 0
     print.printer_EPSON_Stylus_Photo_R1800.print_edge_top: 0
     print.printer_EPSON_Stylus_Photo_R1800.print_evenpages: true
     print.printer_EPSON_Stylus_Photo_R1800.print_footercenter:
     print.printer_EPSON_Stylus_Photo_R1800.print_footerleft: &PT
     print.printer_EPSON_Stylus_Photo_R1800.print_footerright: &D
     print.printer_EPSON_Stylus_Photo_R1800.print_headercenter:
     print.printer_EPSON_Stylus_Photo_R1800.print_headerleft: &T
     print.printer_EPSON_Stylus_Photo_R1800.print_headerright: &U
     print.printer_EPSON_Stylus_Photo_R1800.print_in_color: true
     print.printer_EPSON_Stylus_Photo_R1800.print_margin_bottom: 0.5
     print.printer_EPSON_Stylus_Photo_R1800.print_margin_left: 0.5
     print.printer_EPSON_Stylus_Photo_R1800.print_margin_right: 0.5
     print.printer_EPSON_Stylus_Photo_R1800.print_margin_top: 0.5
     print.printer_EPSON_Stylus_Photo_R1800.print_oddpages: true
     print.printer_EPSON_Stylus_Photo_R1800.print_orientation: 0
     print.printer_EPSON_Stylus_Photo_R1800.print_pagedelay: 500
     print.printer_EPSON_Stylus_Photo_R1800.print_paper_data: 1
     print.printer_EPSON_Stylus_Photo_R1800.print_paper_height: 11.00
     print.printer_EPSON_Stylus_Photo_R1800.print_paper_size_type: 0
     print.printer_EPSON_Stylus_Photo_R1800.print_paper_size_unit: 0
     print.printer_EPSON_Stylus_Photo_R1800.print_paper_width: 8.50
     print.printer_EPSON_Stylus_Photo_R1800.print_reversed: false
     print.printer_EPSON_Stylus_Photo_R1800.print_scaling: 1.00
     print.printer_EPSON_Stylus_Photo_R1800.print_shrink_to_fit: true
     print.printer_EPSON_Stylus_Photo_R1800.print_to_file: false
     print.printer_EPSON_Stylus_Photo_R1800.print_unwriteable_margin_bottom: 0
     print.printer_EPSON_Stylus_Photo_R1800.print_unwriteable_margin_left: 0
     print.printer_EPSON_Stylus_Photo_R1800.print_unwriteable_margin_right: 0
     print.printer_EPSON_Stylus_Photo_R1800.print_unwriteable_margin_top: 0
     print.printer_EPSON_XP-410_Series.print_bgcolor: false
     print.printer_EPSON_XP-410_Series.print_bgimages: false
     print.printer_EPSON_XP-410_Series.print_colorspace:
     print.printer_EPSON_XP-410_Series.print_command:
     print.printer_EPSON_XP-410_Series.print_downloadfonts: false
     print.printer_EPSON_XP-410_Series.print_duplex: -925777384
     print.printer_EPSON_XP-410_Series.print_edge_bottom: 0
     print.printer_EPSON_XP-410_Series.print_edge_left: 0
     print.printer_EPSON_XP-410_Series.print_edge_right: 0
     print.printer_EPSON_XP-410_Series.print_edge_top: 0
     print.printer_EPSON_XP-410_Series.print_evenpages: true
     print.printer_EPSON_XP-410_Series.print_footercenter:
     print.printer_EPSON_XP-410_Series.print_footerleft: &PT
     print.printer_EPSON_XP-410_Series.print_footerright: &D
     print.printer_EPSON_XP-410_Series.print_headercenter:
     print.printer_EPSON_XP-410_Series.print_headerleft: &T
     print.printer_EPSON_XP-410_Series.print_headerright: &U
     print.printer_EPSON_XP-410_Series.print_in_color: true
     print.printer_EPSON_XP-410_Series.print_margin_bottom: 0.5
     print.printer_EPSON_XP-410_Series.print_margin_left: 0.5
     print.printer_EPSON_XP-410_Series.print_margin_right: 0.5
     print.printer_EPSON_XP-410_Series.print_margin_top: 0.5
     print.printer_EPSON_XP-410_Series.print_oddpages: true
     print.printer_EPSON_XP-410_Series.print_orientation: 0
     print.printer_EPSON_XP-410_Series.print_page_delay: 50
     print.printer_EPSON_XP-410_Series.print_paper_data: 1
     print.printer_EPSON_XP-410_Series.print_paper_height: 11.00
     print.printer_EPSON_XP-410_Series.print_paper_name:
     print.printer_EPSON_XP-410_Series.print_paper_size_type: 0
     print.printer_EPSON_XP-410_Series.print_paper_size_unit: 0
     print.printer_EPSON_XP-410_Series.print_paper_width: 8.50
     print.printer_EPSON_XP-410_Series.print_plex_name:
     print.printer_EPSON_XP-410_Series.print_resolution: 89273632
     print.printer_EPSON_XP-410_Series.print_resolution_name:
     print.printer_EPSON_XP-410_Series.print_reversed: false
     print.printer_EPSON_XP-410_Series.print_scaling: 1.00
     print.printer_EPSON_XP-410_Series.print_shrink_to_fit: true
     print.printer_EPSON_XP-410_Series.print_to_file: false
     print.printer_EPSON_XP-410_Series.print_unwriteable_margin_bottom: 0
     print.printer_EPSON_XP-410_Series.print_unwriteable_margin_left: 0
     print.printer_EPSON_XP-410_Series.print_unwriteable_margin_right: 0
     print.printer_EPSON_XP-410_Series.print_unwriteable_margin_top: 0

 Graphics

     Adapter Description: S3 Graphics ProSavageDDR
     Vendor ID: 0x5333
     Device ID: 0x8d04
     Adapter RAM: Unknown
     Adapter Drivers: s3gnb
     Driver Version: 6.14.10.25
     Driver Date: 3-2-2004
     WebGL Renderer: Blocked for your graphics card because of unresolved driver issues.
     GPU Accelerated Windows: 0. Blocked for your graphics card because of unresolved driver issues.

     AzureCanvasBackend: skia
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: none

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.10.2
     4.10.2

     NSS
     3.15.4 Basic ECC
     3.15.4 Basic ECC

     NSS Util
     3.15.4
     3.15.4

     NSS SSL
     3.15.4 Basic ECC
     3.15.4 Basic ECC

     NSS S/MIME
     3.15.4 Basic ECC
     3.15.4 Basic ECC

this is the issue

INCOMING: account2, ,

(imap) imap.cs.com:143, plain, passwordCleartext

OUTGOING: smtp-server.cfl.rr.com:25, plain, none, true

mail from a CS.com account is being sent through a roadrunner mail server.

Your will need to get the SMTP server settings from CS and use their server to send mail.

Tools menu (Alt+T) > Account settings > outgoing server(SMTP) and add an SMTP server.

Then click on the CS account in the account list and select the newly added SMTP server for use by that account.