There are a number of complains against ATT and Yahoo with that errornumber. They can't read or understand your username or pw Mostly they say its on their side. But send us copy of troubleshooting information (under help) Copy text to clipboard and paste it here. There should be no personal details (unless you marked include account names )

Is the following info what you asked for?

TMHH

 Application Basics

   Name: Thunderbird
   Version: 31.4.0
   User Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20150109111741
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) inbound.att.net:995, SSL, passwordCleartext
     OUTGOING: outbound.att.net:465, SSL, passwordCleartext, true

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

 Crash Reports
   http://crash-stats.mozilla.com/report/index/bp-088ac3b3-168a-4bf9-a888-3c8032150219 (2/19/2015)

 Extensions

 Important Modified Preferences

   Name: Value

     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.name.monospace.el: Consolas
     font.name.monospace.tr: Consolas
     font.name.monospace.x-baltic: Consolas
     font.name.monospace.x-central-euro: 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.tr: Calibri
     font.name.sans-serif.x-baltic: Calibri
     font.name.sans-serif.x-central-euro: 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.tr: Cambria
     font.name.serif.x-baltic: Cambria
     font.name.serif.x-central-euro: 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.tr: 14
     font.size.fixed.x-baltic: 14
     font.size.fixed.x-central-euro: 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.tr: 17
     font.size.variable.x-baltic: 17
     font.size.variable.x-central-euro: 17
     font.size.variable.x-cyrillic: 17
     font.size.variable.x-unicode: 17
     font.size.variable.x-western: 20
     gfx.direct3d.last_used_feature_level_idx: 0
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: f7b94541-f48c-4593-9cc9-a4659363199
     network.cookie.prefsMigrated: true
     places.database.lastMaintenance: 1424408023
     places.history.expiration.transient_current_max_pages: 104858
     plugin.importedState: true
     print.printer_Canon_MG5200_series.print_bgcolor: false
     print.printer_Canon_MG5200_series.print_bgimages: false
     print.printer_Canon_MG5200_series.print_colorspace:
     print.printer_Canon_MG5200_series.print_command:
     print.printer_Canon_MG5200_series.print_downloadfonts: false
     print.printer_Canon_MG5200_series.print_duplex: 2097268
     print.printer_Canon_MG5200_series.print_edge_bottom: 0
     print.printer_Canon_MG5200_series.print_edge_left: 0
     print.printer_Canon_MG5200_series.print_edge_right: 0
     print.printer_Canon_MG5200_series.print_edge_top: 0
     print.printer_Canon_MG5200_series.print_evenpages: true
     print.printer_Canon_MG5200_series.print_footercenter:
     print.printer_Canon_MG5200_series.print_footerleft: &PT
     print.printer_Canon_MG5200_series.print_footerright: &D
     print.printer_Canon_MG5200_series.print_headercenter:
     print.printer_Canon_MG5200_series.print_headerleft: &T
     print.printer_Canon_MG5200_series.print_headerright: &U
     print.printer_Canon_MG5200_series.print_in_color: true
     print.printer_Canon_MG5200_series.print_margin_bottom: 0.5
     print.printer_Canon_MG5200_series.print_margin_left: 0.5
     print.printer_Canon_MG5200_series.print_margin_right: 0.5
     print.printer_Canon_MG5200_series.print_margin_top: 0.5
     print.printer_Canon_MG5200_series.print_oddpages: true
     print.printer_Canon_MG5200_series.print_orientation: 0
     print.printer_Canon_MG5200_series.print_page_delay: 50
     print.printer_Canon_MG5200_series.print_paper_data: 1
     print.printer_Canon_MG5200_series.print_paper_height: 11.00
     print.printer_Canon_MG5200_series.print_paper_name:
     print.printer_Canon_MG5200_series.print_paper_size_type: 0
     print.printer_Canon_MG5200_series.print_paper_size_unit: 0
     print.printer_Canon_MG5200_series.print_paper_width: 8.50
     print.printer_Canon_MG5200_series.print_plex_name:
     print.printer_Canon_MG5200_series.print_resolution: 6488165
     print.printer_Canon_MG5200_series.print_resolution_name:
     print.printer_Canon_MG5200_series.print_reversed: false
     print.printer_Canon_MG5200_series.print_scaling: 1.00
     print.printer_Canon_MG5200_series.print_shrink_to_fit: true
     print.printer_Canon_MG5200_series.print_to_file: false
     print.printer_Canon_MG5200_series.print_unwriteable_margin_bottom: 0
     print.printer_Canon_MG5200_series.print_unwriteable_margin_left: 0
     print.printer_Canon_MG5200_series.print_unwriteable_margin_right: 0
     print.printer_Canon_MG5200_series.print_unwriteable_margin_top: 0

 Graphics

     Adapter Description: Intel(R) HD Graphics 4400
     Vendor ID: 0x8086
     Device ID: 0x041e
     Adapter RAM: Unknown
     Adapter Drivers: igdumdim64 igd10iumd64 igd10iumd64 igdumdim32 igd10iumd32 igd10iumd32
     Driver Version: 10.18.10.3939
     Driver Date: 9-8-2014
     Direct2D Enabled: true
     DirectWrite Enabled: true (6.3.9600.17415)
     ClearType Parameters: ClearType parameters not found
     WebGL Renderer: false
     GPU Accelerated Windows: 1/1 Direct3D 10

     AzureCanvasBackend: direct2d
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: direct2d

 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

From what I can see your setup is correct. You stated it's a intermittent problem so your password and username must be correct. ATT just don't like you some days.

(#AUTH201) Service is not available; please try again later. 
(#IS6532) internal server error.
Yahoo and ATT didn't respond or said they were working on it (for years).

The best advice I found that helped somebody was to change your password.

I tried changing my password with no results. ATT said they couldn't help unless I wanted to pay for a remote service they offer which I declined.

Any other suggestions?

Thanks, TMHH

The error says internal server error. AT&Ts server issued that error code. Thunderbird did not make it up. Sounds like AT&T has a bad server in their server rack and you have issues when you get that server. Other times you get a serve that is working. Not a Thunderbird problem.

It seems to be better; I have no idea why.

Thanks for your help, TMHH