The error message is from your email provider's server. You'll need to ask them why they classify your message as spam.

You may have missed my description. My email provider, Optimum, has no problem when I send the same email reply through their web-based client.

I haven't missed your description. Webmail is entirely different than sending mail via SMTP.

christ1 said

I haven't missed your description. Webmail is entirely different than sending mail via SMTP.

Still following up on this. It's gotten more bizarre. Thunderbird is definitely somehow involved, as I now also see the same basic issue when I send email from my gmail account to an optimum email address. In this case the mail does go out but bounces back with the same error. In summary, any email I send from my optimum address or to an optimum address using TB triggers their spam blocker, but not when sent from webmail clients. Optimum tech support has finally understood me and has escalated its investigation (still waiting). Wondering if someone here has any thoughts on what TB may be contributing to the spam triggering.

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.

Thanks very much for your willingness to look into it. Here's the info (went over the character limit, so I removed all the printer-related lines):

 Application Basics

   Name: Thunderbird
   Version: 45.6.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20161222195647
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) mail.optimum.net:110, plain, passwordCleartext
     OUTGOING: , mail.optimum.net:25, plain, passwordCleartext, true

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

   account3:
     INCOMING: account3, , (pop3) mail.dumka.org:110, plain, passwordCleartext
     OUTGOING: , host285.hostmonster.com:465, plain, passwordCleartext, true

   account6:
     INCOMING: account6, , (pop3) host285.hostmonster.com:995, SSL, passwordCleartext
     OUTGOING: , host285.hostmonster.com:465, plain, passwordCleartext, true

   account8:
     INCOMING: account8, , (pop3) mail.optimum.net:110, plain, passwordCleartext
     OUTGOING: , mail.optimum.net:587, plain, passwordCleartext, true

   account11:
     INCOMING: account11, , (pop3) pop.googlemail.com:995, SSL, passwordCleartext
     OUTGOING: , smtp.googlemail.com:465, SSL, passwordCleartext, true

   account12:
     INCOMING: account12, , (pop3) mail.optimum.net:110, plain, passwordCleartext
     OUTGOING: , mail.optimum.net:587, plain, passwordCleartext, true

 Crash Reports

 Extensions
   Lightning, 4.7.6, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 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
     browser.display.background_color: #FFFFCC
     browser.display.foreground_color: #663366
     browser.display.screen_resolution: 96
     dom.apps.reset-permissions: true
     extensions.lastAppVersion: 45.6.0
     font.internaluseonly.changed: true
     font.minimum-size.x-western: 14
     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: Century Gothic
     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
     gfx.crash-guard.glcontext.appVersion: 45.5.1
     gfx.crash-guard.glcontext.deviceID: 0x06e4
     gfx.crash-guard.glcontext.driverVersion: 9.18.13.4195
     gfx.crash-guard.glcontext.gfx.driver-init.direct3d11-angle: true
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle: true
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-force-d3d11: false
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-force-warp: false
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-try-d3d11: true
     gfx.crash-guard.status.glcontext: 2
     gfx.direct3d.last_used_feature_level_idx: 1
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: c271bd16-05e3-4d53-9c35-b2525790791
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"21"},"attachmentCol":{"visible":fals…
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1483047334
     places.history.expiration.transient_current_max_pages: 104858
     places.history.expiration.transient_optimal_database_size: 167772160
     plugin.disable_full_page_plugin_for_types: application/pdf
     plugin.importedState: true
     print.print_printer: Adobe PDF

OUTGOING: , mail.optimum.net:25, plain, passwordCleartext, true

Some email providers block port 25 to prevent spammers from using it. Check with your email provider about the required settings for the outgoing server.

From their web site at https://support.mozilla.org/en-US/questions/1152031 :

SMTP (Outgoing) Server: mail.optimum.net Port: 587 Check 'My SMTP server requires authentication'

They neglected to mention anything about 'Connection security', so I'd first try 'StartTLS'. If that doesn't work try 'none'.

To get to the Account Settings: At the top right of the Thunderbird window, click the menu button > Options > Account Settings

Adjust the settings for the Outgoing Server (SMTP) at the bottom of the left pane.

In any case, the hero's at optimum.net should explain that to you.

Thanks for your suggestions. Tried following them, but don't see an item 'My SMTP server requires authentication' that you mention. Attached is a screen shot of the choices in my SMTP settings window. I've tried several combination and either the same error or different ones, like "encryption not supported" when I tried that. Nothing's changed in those settings in years, BTW. I'll probably just have to wait and see what the optimum folks come up with. But your efforts are very much appreciated.

"Normal password" means authentication. The warning about encryption suggests that you should change STARTTLS to "none".