Why did you change the server settings in the first place?

Did you check with your email provider about the correct settings? What are the correct settings?

Can you post your Troubleshooting Information?

AppMenu button - Help - Troubleshooting Information

chrsit1 thank you very much for answering !

in a message to all the users, my provider suggested to switch the settings of the network security from "None" to "SSL/TLS" for both incoming/outgoing servers.

here are my troubleshot info

Name Thunderbird Version 24.4.0 User Agent Mozilla/5.0 (Windows NT 6.0; rv:24.0) Gecko/20100101 Thunderbird/24.4.0 Profile Folder Show Folder (Local drive) Application Build ID 20140316131045 Enabled Plugins about:plugins Build Configuration about:buildconfig Crash Reports about:crashes Memory Use about:memory Mail and News Accounts ID Incoming server Outgoing servers Name Connection security Authentication method Name Connection security Authentication method Default? account1 (none) Local Folders plain passwordCleartext account2 (pop3) pop.onlinehome.de:110 plain passwordCleartext auth.mail.onlinehome.de:1 plain none true Extensions Name Version Enabled ID CompactHeader 2.0.8 true {58D4392A-842E-11DE-B51A-C7B855D89593} Extra Folder Columns 1.1.5 true extra-cols@jminta_gmail.com Test Pilot for Thunderbird 1.3.9 true tbtestpilot@labs.mozilla.com Important Modified Preferences Name Value accessibility.typeaheadfind.flashBar 0 browser.cache.disk.capacity 358400 browser.cache.disk.smart_size.first_run false browser.cache.disk.smart_size.use_old_max false browser.cache.disk.smart_size_cached_value 358400 browser.zoom.full true extensions.lastAppVersion 24.4.0 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.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 14 gfx.blacklist.suggested-driver-version 10.6 mail.openMessageBehavior.version 1 mail.winsearch.enable true mail.winsearch.firstRunDone true mail.winsearch.global_reindex_time 1304330180 mailnews.database.global.datastore.id 1f751938-6014-4a72-936c-244369b16e3 network.cookie.lifetimePolicy 1 network.cookie.prefsMigrated true places.database.lastMaintenance 1395677004 places.history.expiration.transient_current_max_pages 50264 places.history.expiration.transient_optimal_database_size 80425778 plugin.importedState true print.print_printer Brother MFC-235C Printer print.printer_Brother_MFC-235C_Printer.print_bgcolor false print.printer_Brother_MFC-235C_Printer.print_bgimages false print.printer_Brother_MFC-235C_Printer.print_command print.printer_Brother_MFC-235C_Printer.print_downloadfonts true print.printer_Brother_MFC-235C_Printer.print_evenpages true print.printer_Brother_MFC-235C_Printer.print_footercenter print.printer_Brother_MFC-235C_Printer.print_footerleft &PT print.printer_Brother_MFC-235C_Printer.print_footerright &D print.printer_Brother_MFC-235C_Printer.print_headercenter print.printer_Brother_MFC-235C_Printer.print_headerleft &T print.printer_Brother_MFC-235C_Printer.print_headerright &U print.printer_Brother_MFC-235C_Printer.print_in_color true print.printer_Brother_MFC-235C_Printer.print_margin_bottom 0.5 print.printer_Brother_MFC-235C_Printer.print_margin_left 0.5 print.printer_Brother_MFC-235C_Printer.print_margin_right 0.5 print.printer_Brother_MFC-235C_Printer.print_margin_top 0.5 print.printer_Brother_MFC-235C_Printer.print_oddpages true print.printer_Brother_MFC-235C_Printer.print_orientation 0 print.printer_Brother_MFC-235C_Printer.print_pagedelay 500 print.printer_Brother_MFC-235C_Printer.print_paper_data 9 print.printer_Brother_MFC-235C_Printer.print_paper_height 11,00 print.printer_Brother_MFC-235C_Printer.print_paper_size 0 print.printer_Brother_MFC-235C_Printer.print_paper_size_type 0 print.printer_Brother_MFC-235C_Printer.print_paper_size_unit 1 print.printer_Brother_MFC-235C_Printer.print_paper_width 8,50 print.printer_Brother_MFC-235C_Printer.print_reversed false print.printer_Brother_MFC-235C_Printer.print_scaling 1,00 print.printer_Brother_MFC-235C_Printer.print_shrink_to_fit true print.printer_Brother_MFC-235C_Printer.print_to_file false Graphics Adapter Description ATI Radeon Xpress 1200 Vendor ID 0x1002 Device ID 0x791f Adapter RAM Unknown Adapter Drivers atiumdag atiumdva atitmmxx Driver Version 8.342.0.0 Driver Date 2-2-2007 Direct2D Enabled Blocked for your graphics driver version. Try updating your graphics driver to version 10.6 or newer. DirectWrite Enabled false (7.0.6002.23200) ClearType Parameters ClearType parameters not found WebGL Renderer Blocked for your graphics driver version. Try updating your graphics driver to version 10.6 or newer. GPU Accelerated Windows 0. Blocked for your graphics driver version. Try updating your graphics driver to version 10.6 or newer. AzureCanvasBackend skia AzureFallbackCanvasBackend cairo AzureContentBackend none JavaScript Incremental GC 1 Accessibility Activated 0 Prevent Accessibility 0 Library Versions Expected minimum version Version in use NSPR 4.10.2 4.10.2 NSS 3.15.4 Basic ECC 3.15.4 Basic ECC NSS Util 3.15.4 3.15.4 NSS SSL 3.15.4 Basic ECC 3.15.4 Basic ECC NSS S/MIME 3.15.4 Basic ECC 3.15.4 Basic ECC

This website messes up the settings data horribly. It looks like your smtp is set up as:

auth.mail.onlinehome.de:1

That's not a likely port, and I see no sign of SSL being enabled. TLS/SSL on smtp normally uses 465, or possibly 587 or 25 with STARTTLS.

Can you also post the exact instructions and settings you received from your provider?

Turning on SSL/TLS is good, but you possibly did do something wrong with the SMTP server settings.