Some info here: http://kb.mozillazine.org/Connection_errors_-_SMTP#Connecting

suggest check computer hostname

My Mac's hostname was Macintosh.local Not sure if it's supposed to be something different than that.

The page you referenced also made mention of another link: http://forums.mozillazine.org/viewtopic.php?t=639883#3303828 where one person suggested altering the config editor. I did has he said but I think it might have made things worse. Now I get a different error (something about not being able to find my SMtP server. Going in circles here.

Whose smtp server are you using? You mentioned your ISP so I guess you're using the email account provided with your internet connection.

In order for us to better investigate this issue you need to add a little more non-personal information.

Please add the troubleshooting information to your post. To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help);
• Choose Troubleshooting Information;
• Clear the check box Include account names;
• Use the button Copy to clipboard to select all;
• Paste this in your reply here.

I would never advocate using the config editor to adjust account server settings. Just about all the adjustments you need to make are accessible via the GUI.

You might have a problem that we occasionally encounter with some servers. An email message contains records of servers that have handled the message. Thunderbird inserts the local IP address (yours) and some servers object to this, taking the view (erroneous in my understanding of it) that local or non-routeable addresses are indicative of spam or "owned" machines.

It's possible to get Thunderbird to lie about the origin of messages if it is really necessary, and this is covered in the article linked earlier.

I think I figured it out. I followed your advice above (trouble shooting information) and noticed on the outgoing server name, it was labelled "mail.mcsnet.ca:25". I have two mail accounts, one from Telus, and the other from MCSnet. I've always used the Telus account and have sort of forgotten about the MCSnet account. However, the name "mail.mcsnet.ca:25" was named under the Telus account. When the Telus account was working properly before, in the past, I could never get the MCSnet outgoing mail to work (would never send out mail) so that's why I sort of put it out of my mind. But when I switched the default account to MCSnet (from Telus). I now noticed the MCSnet outgoing mail was working great. But the Telus outgoing mail would still not work until I deleted the outgoing account, now I can send out mail from my Telus account (using the MCSnet account). Not sure how the name "mail.mcsnet.ca:25" got labelled to the Telus account but somehow it did. Anyway, I'm sure if I deleted the MCSnet account and used only the Telus outgoing mail, it would probably work fine too.

Thanks for all your help.

My mail was working fine until latest update to 52.3.0. now it fails with sending of message failed. Out going server unknown. I've run the troubleshooter for thunderbird and its as follows:

 Application Basics

   Name: Thunderbird
   Version: 52.3.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20170815040357
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account5:
     INCOMING: account5, , (pop3) mail.hoosierinternet.com:110, alwaysSTARTTLS, passwordEncrypted
     OUTGOING: , mail.hoosierinternet.com:587, alwaysSTARTTLS, passwordEncrypted, true

   account9:
     INCOMING: account9, , (imap) mail.hoosierinternet.com:143, alwaysSTARTTLS, passwordEncrypted
     OUTGOING: , smtp.hoosierinternet.com:587, alwaysSTARTTLS, passwordEncrypted, true

 Crash Reports
   https://crash-stats.mozilla.com/report/index/bp-c6727e03-f666-472a-bf8e-b16672161014 (10/14/2016)

 Extensions
   Lightning, 5.4.3, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     accessibility.lastLoadDate: 1506437948
     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: 52.3.0
     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: 17
     gfx.direct3d.last_used_feature_level_idx: 0
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: e8c9ece1-6987-40c9-9af9-73fbb8ac179
     media.gmp.storage.version.observed: 1
     network.cookie.cookieBehavior: 1
     network.cookie.lifetimePolicy: 2
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1506435667
     places.history.expiration.transient_current_max_pages: 122334
     plugin.importedState: true
     plugin.state.flash: 0
     plugin.state.npdeployjava: 0
     plugin.state.npfacebookvideocalling: 0
     plugin.state.npfoxitreaderplugin: 2
     plugin.state.npintelwebapiipt: 0
     plugin.state.npintelwebapiupdater: 2
     plugin.state.npsitesafety: 0
     print.printer_HP_Officejet_Pro_8600_(Network).print_bgcolor: false
     print.printer_HP_Officejet_Pro_8600_(Network).print_bgimages: false
     print.printer_HP_Officejet_Pro_8600_(Network).print_evenpages: true
     print.printer_HP_Officejet_Pro_8600_(Network).print_footercenter:
     print.printer_HP_Officejet_Pro_8600_(Network).print_footerleft: &PT
     print.printer_HP_Officejet_Pro_8600_(Network).print_footerright: &D
     print.printer_HP_Officejet_Pro_8600_(Network).print_headercenter:
     print.printer_HP_Officejet_Pro_8600_(Network).print_headerleft: &T
     print.printer_HP_Officejet_Pro_8600_(Network).print_headerright: &U
     print.printer_HP_Officejet_Pro_8600_(Network).print_in_color: true
     print.printer_HP_Officejet_Pro_8600_(Network).print_margin_bottom: 0.5
     print.printer_HP_Officejet_Pro_8600_(Network).print_margin_left: 0.5
     print.printer_HP_Officejet_Pro_8600_(Network).print_margin_right: 0.5
     print.printer_HP_Officejet_Pro_8600_(Network).print_margin_top: 0.5
     print.printer_HP_Officejet_Pro_8600_(Network).print_oddpages: true
     print.printer_HP_Officejet_Pro_8600_(Network).print_orientation: 0
     print.printer_HP_Officejet_Pro_8600_(Network).print_reversed: false
     print.printer_HP_Officejet_Pro_8600_(Network).print_scaling: 1.00
     print.printer_HP_Officejet_Pro_8600_(Network).print_shrink_to_fit: false
     privacy.donottrackheader.enabled: true
     security.sandbox.content.tempDirSuffix: {d058173c-8da3-4b75-88b1-0dea9947dae4}

 Graphics

     GPU #1
     Description: AMD Radeon HD 7700 Series
     Vendor ID: 0x1002
     Device ID: 0x683d
     RAM: 2048
     Drivers: aticfx64 aticfx64 aticfx64 amdxc64 aticfx32 aticfx32 aticfx32 amdxc32 atiumd64 atidxx64 atidxx64 atiumdag atidxx32 atidxx32 atiumdva atiumd6a atitmm64
     Driver Version: 22.19.162.4
     Driver Date: 4-24-2017

     Features
     Direct2D: false
     DirectWrite: true (10.0.15063.413)
     ClearType Parameters: Gamma: 2.2 Pixel Structure: BGR ClearType Level: 0 Enhanced Contrast: 400
     WebGL Renderer: Google Inc. -- ANGLE (AMD Radeon HD 7700 Series 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
     3.28.5
     3.28.5

     NSS Util
     3.28.5
     3.28.5

     NSS SSL
     3.28.5
     3.28.5

     NSS S/MIME
     3.28.5
     3.28.5