Did you contact Yahoo about your problem? There was a major Yahoo outage about a week ago. They may still not have fully recovered yet.

Thanks for this information, but I supposed that since I normally receive my emails through web and android, the issue would rather be related to Thunderbird than Yahoo.

If this assumption is wrong, I surely should contact Yahoo.

Webmail is something entirely different than using email via POP, IMAP, and SMTP. You haven't mentioned whether you're using a real email client on the phone, or the Yahoo mail app. In case of the latter presumably that's just like using webmail.

No, I am not using Yahoo mail app on the phone, I am using a real email client (K-9 Mail). Furthermore, I just installed a different email client on my pc and works fine with yahoo accounts.

Please post your Troubleshooting Information. At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information. Press the "Copy text to clipboard button" and paste the information into your reply.

Application Basics

   Name: Thunderbird
   Version: 52.6.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20180123185941
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account1:
     INCOMING: account1, , (imap) imap.med.uoc.gr:993, SSL, passwordCleartext
     OUTGOING: , mail.med.uoc.gr:465, SSL, passwordCleartext, true

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account4:
     INCOMING: account4, , (pop3) pop3.teemail.gr:110, plain, passwordCleartext
     OUTGOING: , mail-out.cyta.gr:587, plain, none, true

   account6:
     INCOMING: account6, , (pop3) pop.mail.yahoo.com:995, SSL, passwordCleartext
     OUTGOING: , smtp.mail.yahoo.com:465, SSL, passwordCleartext, true

   account7:
     INCOMING: account7, , (pop3) incoming.tee.gr:110, plain, passwordCleartext
     OUTGOING: , outgoing.tee.gr:465, SSL, passwordCleartext, true

   account8:
     INCOMING: account8, , (pop3) pop.mail.yahoo.com:995, SSL, passwordCleartext
     OUTGOING: , smtp.mail.yahoo.com:465, SSL, passwordCleartext, true

   account10:
     INCOMING: account10, , (imap) imap.uoc.gr:993, SSL, passwordCleartext
     OUTGOING: , mta.uoc.gr:587, alwaysSTARTTLS, passwordCleartext, true

   account12:
     INCOMING: account12, , (pop3) pop.gmail.com:995, SSL, passwordCleartext
     OUTGOING: , smtp.gmail.com:465, SSL, passwordCleartext, true

 Crash Reports

 Extensions
   Get All Mail Button Thunderbird, 0.1, true, {0D4D76B0-657A-41b0-B3ED-60C638F4D97D}
   Google Search for Thunderbird, 1.0, true, gsearch@standard8.plus.com
   Lightning, 5.4.6, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   Mark Junk Read, 1.01, true, mark-junk-read@alexcabal.com

 Important Modified Preferences

   Name: Value

     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
     dom.apps.reset-permissions: true
     extensions.lastAppVersion: 52.6.0
     font.internaluseonly.changed: true
     font.name.monospace.el: 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.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.x-cyrillic: Cambria
     font.name.serif.x-unicode: Cambria
     font.name.serif.x-western: Cambria
     font.size.fixed.el: 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.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: e2ac3995-817a-4e42-802f-d84a45c78ae
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     mailnews.database.global.views.global.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     media.gmp.storage.version.observed: 1
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1519244344
     places.history.enabled: false
     places.history.expiration.transient_current_max_pages: 122334
     plugin.importedState: true
     privacy.donottrackheader.enabled: true
     security.sandbox.content.tempDirSuffix: {e42b0b73-c2a6-4501-8fe6-54a3fa27b3b9}

 Graphics

     GPU #1
     Description: Intel(R) HD Graphics 630
     Vendor ID: 0x8086
     Device ID: 0x591b
     RAM: Unknown
     Drivers: igdumdim64 igd10iumd64 igd10iumd64 igd12umd64 igdumdim32 igd10iumd32 igd10iumd32 igd12umd32
     Driver Version: 22.20.16.4836
     Driver Date: 10-17-2017

     GPU #2
     Description: NVIDIA GeForce GTX 1050 Ti
     Vendor ID: 0x10de
     Device ID: 0x1c8c
     RAM: 4096
     Drivers: C:\WINDOWS\System32\DriverStore\FileRepository\nvdm.inf_amd64_a8ceb9685c10caa7\nvldumdx.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nvdm.inf_amd64_a8ceb9685c10caa7\nvldumdx.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nvdm.inf_amd64_a8ceb9685c10caa7\nvldumdx.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nvdm.inf_amd64_a8ceb9685c10caa7\nvldumdx.dll C:\WINDOWS\System32\DriverStore\FileRepository\nvdm.inf_amd64_a8ceb9685c10caa7\nvldumd.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nvdm.inf_amd64_a8ceb9685c10caa7\nvldumd.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nvdm.inf_amd64_a8ceb9685c10caa7\nvldumd.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nvdm.inf_amd64_a8ceb9685c10caa7\nvldumd.dll
     Driver Version: 23.21.13.8816
     Driver Date: 10-30-2017

     Features
     Direct2D: false
     DirectWrite: true (10.0.16299.15)
     ClearType Parameters: Gamma: 2.2 Pixel Structure: BGR ClearType Level: 50 Enhanced Contrast: 300
     WebGL Renderer: Google Inc. -- ANGLE (Intel(R) HD Graphics 630 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
     3.28.6
     3.28.6

     NSS Util
     3.28.6
     3.28.6

     NSS SSL
     3.28.6
     3.28.6

     NSS S/MIME
     3.28.6
     3.28.6

There are two Yahoo accounts, account6, and account8. Which one is having the problem? The server settings look good anyway.

Also make sure to 'Allow apps that use less secure sign in' for your accounts. https://help.yahoo.com/kb/SLN27791.html

The "Allow apps that use less secure sign in" option is already on and the "Two-step verification" option is off.

Both Yahoo accounts have problem, plus one more that I removed and tried to re-Add, but couldn't proceed because of the errors that I have mentioned.

Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/kb/safe-mode

Does the problem go away?

Yes, in safe mode I can receive new mails from my Yahoo accounts and I was able to re-add my other Yahoo account.

But when I return to normal mode, I still can't receive new emails from yahoo accounts.

I figured out that there is a confict with Eset NOD32 Antivirus. When I disable "Application protocol content filtering" (Advanced Setup > WEB AND EMAIL > PROTOCOL FILTERING > Enable application protocol content filtering > x), I can receive new mails from my yahoo accounts. The thing is that while I have changed nothing to my antivirus settings, before 12/02 I could normally receive new mails, and after that date I couldn't.

Now, should I just add Thunderbird to the "Excluded Applications" list of eset? Is it safe?

Now, should I just add Thunderbird to the "Excluded Applications" list of eset? Is it safe?

If that fixes the problem, do it. And yes, it is safe.

Some general suggestions wrt anti-virus software: Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

Thank you for helping me figure out what was wrong and for your suggestions.

But I have two questions:

christ1 said

Don't let your anti-virus software scan incoming messages.
Don't let your anti-virus software scan attachments.

Why?

How will my pc be protected?

The suggested changes disable various features, but you ARE still protected from viruses by other protection features. As long as the real time scan is turned on it will scan all the files and attachments and will alert you if there is any infection.

Thank you