All your SMTP server settings say none for password. Since it is failing on verification you should set that to normal password.

Thank you, but it made no difference. TWC is of no help, so I appreciate any assistance you can provide.

After setting to "normal password" (using the setting password, transmitted insecurely) I get the same error message. In case it's useful, here is the troubleshooting information pulled just now:

 Application Basics

   Name: Thunderbird
   Version: 31.4.0
   User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:31.0) Gecko/20100101 Thunderbird/31.4.0
   Profile Folder: Show in Finder
   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

   account3:
     INCOMING: account3, , (pop3) mail.twc.com:110, plain, passwordCleartext
     OUTGOING: smtp-server.roadrunner.com:587, plain, passwordCleartext, true

   account4:
     INCOMING: account4, , (pop3) mail.twc.com:110, plain, passwordCleartext
     OUTGOING: smtp-server.roadrunner.com:587, plain, passwordCleartext, true

 Crash Reports

 Extensions

 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.internaluseonly.changed: true
     mail.openMessageBehavior.version: 1
     mail.spotlight.firstRunDone: true
     mailnews.database.global.datastore.id: 677c277e-84f2-5245-9c66-47b73a933dc
     network.cookie.prefsMigrated: true
     places.database.lastMaintenance: 1422571014
     places.history.expiration.transient_current_max_pages: 104858
     places.history.expiration.transient_optimal_database_size: 85899344
     plugin.importedState: true
     plugin.state.adobepdfviewernpapi: 0
     plugin.state.divxbrowserplugin: 0
     plugin.state.silverlight: 0

 Graphics

     Vendor ID: 0x8086
     Device ID: 0x a26
     WebGL Renderer: false
     GPU Accelerated Windows: 2/2 OpenGL

     AzureCanvasBackend: quartz
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: none
     AzureContentBackend: quartz

 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

The next thing I would try is setting connection security to STARTTLS. Leave the password at normal for now.

Ok, tried that and got a new error message:

Sending of message failed. An error occurred sending mail: Unable to establish a secure link with SMTP server smtp-server.roadrunner.com using STARTTLS since it doesn't advertise that feature. Switch off STARTTLS for that server or contact your service provider.

I really appreciate the guidance. Thank you.

Set connection security back to none. I keep seeing that they want to see your complete email address for the username. Do you have it setup like that? It also says it can use port 587 or 25. I guess you could try port 25. After that I am not sure what to suggest.

Thanks again. Within Thunderbird I have 2 email accounts. One is XXXXX@roadrunner.com. The other is ZZZZZ@roadrunner.com. It is this second account that has been generating the error.

I only have one SMTP setting available, and it has always worked with XXXXX@roadrunner.com as the user name. Until now.

I switched the user name in the SMTP setting from XXXXX@roadrunner.com to ZZZZZ@roadrunner.com. I can now send from ZZZZZ@roadrunner.com but not from XXXXX@roadrunner.com

I have tried port 25, to no benefit.

Might the problem stem from having the 2 email accounts but only 1 SMTTP setting available? If so, is there a way to get an SMTP setting for each account, instead of one shared between the two?

I read how to have multiple SMTP identities and followed the directions. It now works fine. Thanks again for your help.