SO you have created IMAP mail accounts in a new installation. Is that correct?

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I actually didn't create an IMAP account for my (one) email account, but I see the app has switched from SMTP to IMAP servers without telling me - and is having recurring difficulty connecting. Here's what's on the clipboard:

 Application Basics

   Name: Thunderbird
   Version: 52.7.0
   User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20180322145026
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account1:
     INCOMING: account1, , (imap) imap.mail.yahoo.com:993, SSL, passwordCleartext
     OUTGOING: , smtp.mail.yahoo.com:465, SSL, passwordCleartext, true

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

 Crash Reports

 Extensions
   Lightning, 5.4.7, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     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
     extensions.lastAppVersion: 52.7.0
     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.blacklist.direct2d: 3
     gfx.blacklist.direct2d.failureid: FEATURE_FAILURE_DL_BLACKLIST_g984
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: 93f5034e-337e-448b-94ec-35908a90a85
     media.gmp.storage.version.observed: 1
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1522020591
     places.history.expiration.transient_current_max_pages: 81738
     plugin.importedState: true
     print.printer_HP_Photosmart_C4200_series.print_bgcolor: false
     print.printer_HP_Photosmart_C4200_series.print_bgimages: false
     print.printer_HP_Photosmart_C4200_series.print_colorspace:
     print.printer_HP_Photosmart_C4200_series.print_command:
     print.printer_HP_Photosmart_C4200_series.print_downloadfonts: false
     print.printer_HP_Photosmart_C4200_series.print_duplex: 1515870810
     print.printer_HP_Photosmart_C4200_series.print_edge_bottom: 0
     print.printer_HP_Photosmart_C4200_series.print_edge_left: 0
     print.printer_HP_Photosmart_C4200_series.print_edge_right: 0
     print.printer_HP_Photosmart_C4200_series.print_edge_top: 0
     print.printer_HP_Photosmart_C4200_series.print_evenpages: true
     print.printer_HP_Photosmart_C4200_series.print_footercenter:
     print.printer_HP_Photosmart_C4200_series.print_footerleft:
     print.printer_HP_Photosmart_C4200_series.print_footerright:
     print.printer_HP_Photosmart_C4200_series.print_headercenter:
     print.printer_HP_Photosmart_C4200_series.print_headerleft:
     print.printer_HP_Photosmart_C4200_series.print_headerright:
     print.printer_HP_Photosmart_C4200_series.print_in_color: true
     print.printer_HP_Photosmart_C4200_series.print_margin_bottom: 0.5
     print.printer_HP_Photosmart_C4200_series.print_margin_left: 0.5
     print.printer_HP_Photosmart_C4200_series.print_margin_right: 0.5
     print.printer_HP_Photosmart_C4200_series.print_margin_top: 0.5
     print.printer_HP_Photosmart_C4200_series.print_oddpages: true
     print.printer_HP_Photosmart_C4200_series.print_orientation: 0
     print.printer_HP_Photosmart_C4200_series.print_page_delay: 50
     print.printer_HP_Photosmart_C4200_series.print_paper_data: -1
     print.printer_HP_Photosmart_C4200_series.print_paper_height: 11.00
     print.printer_HP_Photosmart_C4200_series.print_paper_name:
     print.printer_HP_Photosmart_C4200_series.print_paper_size_type: 0
     print.printer_HP_Photosmart_C4200_series.print_paper_size_unit: 0
     print.printer_HP_Photosmart_C4200_series.print_paper_width: 8.50
     print.printer_HP_Photosmart_C4200_series.print_plex_name:
     print.printer_HP_Photosmart_C4200_series.print_resolution: 600
     print.printer_HP_Photosmart_C4200_series.print_resolution_name:
     print.printer_HP_Photosmart_C4200_series.print_reversed: false
     print.printer_HP_Photosmart_C4200_series.print_scaling: 0.90
     print.printer_HP_Photosmart_C4200_series.print_shrink_to_fit: false
     print.printer_HP_Photosmart_C4200_series.print_to_file: false
     print.printer_HP_Photosmart_C4200_series.print_unwriteable_margin_bottom: 0
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     print.printer_HP_Photosmart_C4200_series.print_unwriteable_margin_top: 0
     security.sandbox.content.tempDirSuffix: {0a293ce2-3d87-4ff5-b093-5f83bd3cf68c}

 Graphics

     GPU #1
     Description: Intel(R) G33/G31 Express Chipset Family
     Vendor ID: 0x8086
     Device ID: 0x29c2
     RAM: Unknown
     Drivers: igdumd64 igdumdx32
     Driver Version: 8.15.10.1930
     Driver Date: 9-23-2009

     Features
     Direct2D: Blocked for your graphics driver version.
     DirectWrite: true (6.2.9200.17461)
     WebGL Renderer: Google Inc. -- ANGLE (Software Adapter Direct3D11 vs_5_0 ps_5_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

SMTP is an outgoing mail protocol. You never got mail using it. EVER!

Thunderbird an not change POP account to IMAP. Although I have been fielding questions from people wanting it to do so for the last 6 years.

But all that is irrelevant. You need to create your account as POP in Thunderbird and then it will behave exactly as you appear to think it should.

Note that the online providers make this as difficult as possible as it means they no longer have control of all your email. But here are the instructions they provide for their Australian users (ME) https://au.help.yahoo.com/kb/SLN4724.html

Intermittent problems contacting Yahoo servers appear to be common. I think their service suffers from the same malaise that appears to infect anything they do. These issues are often made worse by anti virus product attempting to scan mail and loosing the plot instead.

Thanks for your help, Matt!

I deleted (with some trepidation) the apparently auto-created IMAP account and created a new POP/SMTP account, which has been behaving well for a week now. Ironically, I found it helpful that my mailboxes were duplicated on the Yahoo site so I could retrieve the messages lost when I deleted the IMAP account. If I could figure out how the IMAP account happened, I could perhaps prevent a recurrence. But at least I now know the symptoms and the cure.

When adding an account to Thunderbird, it will choose IMAP if available. In cases where POP is also available and is advertised, it may offer you the option to choose between them. If the server doesn't advertise POP but you know it is available, then it's up to you to override the automatic account set-up process and insert the POP settings yourself.

Hi, Zenos--

Thanks for your input. What still puzzles me is that I have had a POP/SMTP account in Thunderbird for years, and only recently it apparently decided to convert it to IMAP and my messages got scrambled.

I'm still old-fashioned enough to think that computers and applications should follow instructions instead of doing things on their own.