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.

Here is its.............thanks for helping.

 Application Basics

   Name: Thunderbird
   Version: 52.1.0
   User Agent: Mozilla/5.0 (Windows NT 5.1; rv:52.0) Gecko/20100101 Thunderbird/52.1.0
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20170428151619
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

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

   account2:
     INCOMING: account2, , (pop3) mail.interfold.com:110, plain, passwordCleartext
     OUTGOING: , mail.interfold.com:25, plain, passwordCleartext, true

   account5:
     INCOMING: account5, , (imap) imap.googlemail.com:993, SSL, passwordCleartext
     OUTGOING: , smtp.googlemail.com:465, SSL, passwordCleartext, true
     OUTGOING: , smtp.googlemail.com:465, SSL, passwordCleartext, false
     OUTGOING: , smtp.googlemail.com:465, SSL, passwordCleartext, false
     OUTGOING: , smtp.googlemail.com:465, SSL, passwordCleartext, false

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

 Crash Reports
   https://crash-stats.mozilla.com/report/index/bp-f230c705-d60b-466a-bffc-4826f2120909 (9/9/2012)
   https://crash-stats.mozilla.com/report/index/bp-1e3fb7e3-1518-4682-91c4-b1ddf2120909 (9/9/2012)

 Extensions
   Lightning, 5.4.1, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   messagenotesplus, 1.5.0.0, true, messagenotesplus@mozilla.org
   Personas Plus, 1.8.2, true, personas@christopher.beard
   Mnenhy, 0.8.6.1-signed.1-let-fixed.1-signed, false, {bf591015-b599-4125-9428-3cb746ddca31}
   Stationery, 0.8.6.2, false, {d0e38b3a-0d60-46bf-bf01-83d4ba041015}

 Important Modified Preferences

   Name: Value

     accessibility.lastLoadDate: 1494864761
     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: 52.1.0
     font.internaluseonly.changed: true
     font.name.serif.x-western: Arial
     general.useragent.extra.mnenhy: Mnenhy/0.8.4
     gfx.blacklist.direct2d: 3
     gfx.blacklist.layers.direct3d9: 3
     gfx.blacklist.suggested-driver-version: 6.1400.1000.5218
     gfx.blacklist.webgl.angle: 3
     mail.openMessageBehavior.version: 1
     mailnews.database.global.datastore.id: ec5fe8b8-31ff-4ca9-bd0a-7221d200328
     media.gmp.storage.version.observed: 1
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1494605133
     places.history.expiration.transient_current_max_pages: 84272
     plugin.importedState: true

The account type of your Gmail account(s) is IMAP. With IMAP it's recommended to set the 'Authentication type' to 'OAuth2' for both, the incoming and outgoing server.

At the top right of the Thunderbird window, click the menu button > Options > Account Settings > Server Settings Choose 'OAuth2' from the 'Authentication method' drop-down menu.

Perfect - that fixed it! THANK YOU very much!

I get this often. Its happened again today - all my addresses need verification, by inputing new password. Why does this happen every so often ? I use hotmail, outlook, gmail etc, & it happens to them all.