I'm assuming you are using imap and access via yahoo: https://help.yahoo.com/kb/frontier/SLN4075.html

What server settings are you using? Post info: In Thunderbird Help > Troubleshooting Information click on 'Copy text to clipboard' In this forum question, right click in a 'Post a Reply' text box and select 'Paste' Remove all info on fonts and printers as they as not required.

Do you access imap account via anything other than Thunderbird ? Do you use a phone etc? If you only use thunderbird on one computer, have you logged on to the webmail account and selected to use the less secure apps option ?

I'm not a techie, so I hope I did this correctly.

Server settings: Server Name = imap.mail.yahoo.com Port = 993 Security - SSL/TLS Authentication - normal password

I also have 2 gmail accounts that are accessed through Yahoo. imap, port 993, same settings except both authentication uses OAuth2. Those accounts seem to work OK.

I do use a smartphone and it will not access the ateleng@frontier.com accoun.

Not sure what you mean by you mean by your last sentence.

Also my wife uses TB on a laptop and her account seems to work fine, different account, same settings - imap.mail.yahoo.com, Port 993, SSL/TLS, normal password.

Troubleshooting info......

 Application Basics

   Name: Thunderbird
   Version: 52.2.1
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20170622064432
   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

   account3:
     INCOMING: account3, , (imap) imap.gmail.com:993, SSL, OAuth2
     OUTGOING: , smtp.gmail.com:465, SSL, OAuth2, true

   account4:
     INCOMING: account4, , (imap) imap.gmail.com:993, SSL, OAuth2
     OUTGOING: , smtp.gmail.com:465, SSL, OAuth2, true

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

 Crash Reports

 Extensions
   Lightning, 5.4.2.1, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     accessibility.lastLoadDate: 1502633515
     accessibility.typeaheadfind.flashBar: 0
     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
     browser.zoom.full: true
     dom.apps.reset-permissions: true
     dom.mozApps.used: true
     extensions.lastAppVersion: 52.2.1
    
     gfx.crash-guard.d3d11layers.appVersion: 52.2.1
     gfx.crash-guard.d3d11layers.deviceID: 0x0412
     gfx.crash-guard.d3d11layers.driverVersion: 20.19.15.4531
     gfx.crash-guard.d3d11layers.feature-d2d: true
     gfx.crash-guard.d3d11layers.feature-d3d11: true
     gfx.crash-guard.status.d3d11layers: 2
     gfx.direct2d.disabled: false
     gfx.direct3d.last_used_feature_level_idx: 0
     layers.acceleration.disabled: false
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mail.winsearch.global_reindex_time: 1501602431
     mailnews.database.global.datastore.id: 296fa242-d607-4bb4-ba45-454fe1df9be
     media.gmp.storage.version.observed: 1
     network.cookie.lifetimePolicy: 2
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1502466900
     places.history.expiration.transient_current_max_pages: 122334
     plugin.importedState: true

re : Q: If you only use thunderbird on one computer, have you logged on to the webmail account and selected to use the less secure apps option ? A: I do use a smartphone and it will not access the ateleng@frontier.com accoun. Not sure what you mean by you mean by your last sentence.

Webmail is the method used to access the account via a browser eg: 'Firefox'. frontier yahoo could be this page: https://frontier.yahoo.com/ https://login.frontier.com/webmail

Usually you may need to create a two step verification process to allow access for a phone, if you switch this on, then you will need to create an application specific password which you would use in Thunderbird instead of the password you use to access webmail. https://help.yahoo.com/kb/frontier/sln2080.html

Good info here: https://answers.microsoft.com/en-us/msoffice/forum/msoffice_outlook-mso_win10/yahoo-frontier-outlook/90a528fa-c631-4cba-817c-00e7ac079fb2

All about frontier yahoo email security. Please read this: https://frontier.com/helpcenter/categories/internet/email/email-security-upgrade

I have a similar problem.

I'm getting that error message for one of my three Yahoo accounts. The other two accounts are working fine. All have the same settings in TB, as far as I can determine. I just recently started having problems with the account in TB, and I'm still getting messages for that account just fine on my iPhone. Would love some help with this!

CJ