Forgot to say that prefs.js is in the default folder of Thunderbird

Well given you are removing all of your preferences and settings in Thunderbird it is more than probably you will remove the setting. I suggest however you approach your troubleshooting somewhat differently.

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
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.

Please also add the result of these troubleshooting steps to your post

• Does Thunderbird work in TB Safe mode (see Thunderbird Safe Mode)?
• Do you use anti-virus and firewall software? What is the version?

TB Safe mode works, but the email still cannot be sent. Windows Defender and its firewall.

 Application Basics
   Name: Thunderbird
   Version: 52.7.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; 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
   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account8:
     INCOMING: account8, , (pop3) pop3.netvision.net.il:110, alwaysSTARTTLS, passwordCleartext
     OUTGOING: , mail.netvision.net.il:587, alwaysSTARTTLS, passwordCleartext, true

 Crash Reports
   https://crash-stats.mozilla.com/report/index/bp-37ac10b8-230c-47b2-99d3-3a5bb2160402 (02-Apr-16)
   https://crash-stats.mozilla.com/report/index/bp-497cb007-d881-4c60-baab-3910f2160102 (02-Jan-16)
   https://crash-stats.mozilla.com/report/index/bp-fbd90fb0-ffe6-42be-b259-a18062160102 (02-Jan-16)
   https://crash-stats.mozilla.com/report/index/bp-f983912e-936a-4fab-bfdb-c83672151015 (15-Oct-15)
   https://crash-stats.mozilla.com/report/index/bp-cfecfd00-74ce-4ab9-8e0c-b7a402141205 (05-Dec-14)

 Extensions
   Add Signature, 0.4.2, true, ml@grufo.com
   AddressLabel, 10, true, AddressLabel@Markus
   Android Contacts Sync, 2.1, true, AndroidContactsSync@rogerang.com
   Automatic Dictionary, 1.10.4, true, automatic_dictionary_extension@jordi_beltran.net
   BirdieSync, 3.2.0.1, true, {A69F5EC7-88F0-4902-A15C-E569DFA33C3A}
   Custom Buttons, 0.0.5.8.9, true, custombuttons@xsms.org
   Duplicate Contact Manager, 0.9.2, true, {b4447f60-db9c-11da-a94d-0800200c9a66}
   FiltaQuilla, 1.3.2, true, filtaquilla@mesquilla.com
   Lightning, 5.4.7, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   MyPhoneExplorer, 2.0.19, true, myphoneexplorer@fjsoft.at
   Provider for Google Calendar, 3.3, true, {a62ef8ec-5fdc-40c2-873c-223b8a6925cc}
   S3.Google Translator, 5.35, true, s3google@translator
   Saved Password Editor, 2.10.4, true, savedpasswordeditor@daniel.dawson
   ThunderBrowse, 3.82, true, ThunderBrowse@thunderbrowse.com
   Email signature for Thunderbird - WiseStamp, 3.14.0, false, wisestamp_tb@wisestamp.com
   Signature Switch, 1.6.16, false, {2ab1b709-ba03-4361-abf9-c50b964ff75d}
   WiseStamp, 3.13.31.1-signed, false, wisestamp@wisestamp.com

 Important Modified Preferences

   Name: Value

     accessibility.lastLoadDate: 1526308125
     accessibility.typeaheadfind.flashBar: 0
     browser.cache.disk.capacity: 358400
     browser.cache.disk.filesystem_reported: 1
     browser.cache.disk.hashstats_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
     dom.max_chrome_script_run_time: 0
     dom.mozApps.used: true
     extensions.lastAppVersion: 52.7.0
     font.internaluseonly.changed: false
     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.crash-guard.glcontext.appVersion: 45.8.0
     gfx.crash-guard.glcontext.deviceID: 0x0152
     gfx.crash-guard.glcontext.driverVersion: 10.18.10.4358
     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: 0
     mail.openMessageBehavior.version: 1
     mail.winsearch.enable: true
     mail.winsearch.firstRunDone: true
     mail.winsearch.global_reindex_time: 1378705833
     mailnews.database.global.datastore.id: cfddd3bc-820d-4f14-a10e-0645350b9f4
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     mailnews.database.global.views.global.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     media.gmp.storage.version.observed: 1
     media.webrtc.debug.aec_log_dir: C:\Users\Joop\AppData\Local\Temp
     media.webrtc.debug.log_file: C:\Users\Joop\AppData\Local\Temp\WebRTC.log
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1525667963
     places.history.expiration.transient_current_max_pages: 50761
     plugin.importedState: true
     print.printer_Canon_Inkjet_MX370_series.print_bgcolor: false
     print.printer_Canon_Inkjet_MX370_series.print_bgimages: false
     print.printer_Canon_Inkjet_MX370_series.print_colorspace:
     print.printer_Canon_Inkjet_MX370_series.print_command:
     print.printer_Canon_Inkjet_MX370_series.print_downloadfonts: false
     print.printer_Canon_Inkjet_MX370_series.print_duplex: 0
     print.printer_Canon_Inkjet_MX370_series.print_edge_bottom: 0
     print.printer_Canon_Inkjet_MX370_series.print_edge_left: 0
     print.printer_Canon_Inkjet_MX370_series.print_edge_right: 0
     print.printer_Canon_Inkjet_MX370_series.print_edge_top: 0
     print.printer_Canon_Inkjet_MX370_series.print_evenpages: true
     print.printer_Canon_Inkjet_MX370_series.print_footercenter:
     print.printer_Canon_Inkjet_MX370_series.print_footerleft: &PT
     print.printer_Canon_Inkjet_MX370_series.print_footerright: &D
     print.printer_Canon_Inkjet_MX370_series.print_headercenter:
     print.printer_Canon_Inkjet_MX370_series.print_headerleft: &T
     print.printer_Canon_Inkjet_MX370_series.print_headerright: &U
     print.printer_Canon_Inkjet_MX370_series.print_in_color: true
     print.printer_Canon_Inkjet_MX370_series.print_margin_bottom: 0.5
     print.printer_Canon_Inkjet_MX370_series.print_margin_left: 0.5
     print.printer_Canon_Inkjet_MX370_series.print_margin_right: 0.5
     print.printer_Canon_Inkjet_MX370_series.print_margin_top: 0.5
     print.printer_Canon_Inkjet_MX370_series.print_oddpages: true
     print.printer_Canon_Inkjet_MX370_series.print_orientation: 0
     print.printer_Canon_Inkjet_MX370_series.print_page_delay: 50
     print.printer_Canon_Inkjet_MX370_series.print_paper_data: 1
     print.printer_Canon_Inkjet_MX370_series.print_paper_height: 11.00
     print.printer_Canon_Inkjet_MX370_series.print_paper_name:
     print.printer_Canon_Inkjet_MX370_series.print_paper_size_type: 0
     print.printer_Canon_Inkjet_MX370_series.print_paper_size_unit: 0
     print.printer_Canon_Inkjet_MX370_series.print_paper_width: 8.50
     print.printer_Canon_Inkjet_MX370_series.print_plex_name:
     print.printer_Canon_Inkjet_MX370_series.print_resolution: 0
     print.printer_Canon_Inkjet_MX370_series.print_resolution_name:
     print.printer_Canon_Inkjet_MX370_series.print_reversed: false
     print.printer_Canon_Inkjet_MX370_series.print_scaling: 1.00
     print.printer_Canon_Inkjet_MX370_series.print_shrink_to_fit: true
     print.printer_Canon_Inkjet_MX370_series.print_to_file: false
     print.printer_Canon_Inkjet_MX370_series.print_unwriteable_margin_bottom: 0
     print.printer_Canon_Inkjet_MX370_series.print_unwriteable_margin_left: 0
     print.printer_Canon_Inkjet_MX370_series.print_unwriteable_margin_right: 0
     print.printer_Canon_Inkjet_MX370_series.print_unwriteable_margin_top: 0
     print.printer_Canon_MX370_series_Printer.print_bgcolor: false
     print.printer_Canon_MX370_series_Printer.print_bgimages: false
     print.printer_Canon_MX370_series_Printer.print_colorspace:
     print.printer_Canon_MX370_series_Printer.print_command:
     print.printer_Canon_MX370_series_Printer.print_downloadfonts: false
     print.printer_Canon_MX370_series_Printer.print_duplex: 0
     print.printer_Canon_MX370_series_Printer.print_edge_bottom: 0
     print.printer_Canon_MX370_series_Printer.print_edge_left: 0
     print.printer_Canon_MX370_series_Printer.print_edge_right: 0
     print.printer_Canon_MX370_series_Printer.print_edge_top: 0
     print.printer_Canon_MX370_series_Printer.pr

Dear Matt. As you might have noticed I replied to your request on 12 May for more information. Could you use it to find the culprit. I must add that on other email programs, such as Windows Mail, sending email went fluently. So the culprit must be in the profile file. I may have to send you the complete profile file (prefs.js)

To Matt You asked for more information, that I sent you. I did not see any reply. Please help (or at least tell me if you cannot help)

I am unable to locate a real netvision web site for settings.

But this site.http://smtpimap.email/netvision.net.il-smtp-imap-email-settings.html

That site indicates just about everything in your account settings is incorrect.

Based on the ancient ports in use I would start by turning off connection security. Set it to none. If that does not work change the server name to those suggested there unless you can get more definitive information from your provider. But you problem is almost certainly those settings are incorrect.

Thanks for giving the site. The server port in my settings was wrong. Several places in the internet gave 587 as incoming port, meantioning that 25 is closed. The site that you mentioned (http://smtpimap.email/netvision.net.il-smtp-imap-email-settings.html ) however gives 25 as the correct port. After changing the setting in my email, sending emails is normal again. (That is to say: They are just sent.)