To understand setup of accounts please post the following information.

In Thunderbird

• Help > Troubleshooting Information
• Do not select checkbox to include account names.
• Click on 'Copy text to clipboard' button

• In this forum question, right click in a 'Post a reply' text box and select 'Paste'
• You can remove/edit all info on fonts and printers, but nothing else.

Allgemeine Informationen

   Name: Thunderbird
   Version: 52.7.0
   User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0
   Profilverzeichnis: Ordner öffnen

             (Lokaler Datenträger)
   Build-ID der Anwendung: 20180326200027
   Aktivierte Plugins: about:plugins
   Build-Konfiguration: about:buildconfig
   Speicherverwendung: about:memory
   Profile: about:profiles

 E-Mail- und Newsgruppen-Konten
   account1:
     INCOMING: account1, , (imap) imap.gmx.net:993, SSL, passwordCleartext
     OUTGOING: , mail.gmx.net:465, SSL, passwordCleartext, true
     OUTGOING: , mail.gmx.net:465, SSL, passwordCleartext, false
     OUTGOING: , mail.gmx.net:465, SSL, passwordCleartext, false
     OUTGOING: , mail.gmx.net:465, SSL, passwordCleartext, false
     OUTGOING: , mail.gmx.net:465, SSL, passwordCleartext, false
     OUTGOING: , mail.gmx.net:465, SSL, passwordCleartext, false

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

   account3:
     INCOMING: account3, , (rss) Feeds, plain, passwordCleartext

 Absturzberichte
   https://crash-stats.mozilla.com/report/index/bp-93a48e6a-f62a-4291-9bcd-d903a0180404 (04.04.2018)

 Erweiterungen
   Change quote and reply format, 0.8.7.1, true, {f6090211-2004-44d8-9090-be3c2adfd66f}
   clamdrib LIN, 0.2.1.9, true, [email protected]
   Lightning, 5.4, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   MoreFunctionsForAddressBook, 1.0b6, true, {3e17310d-82e8-4a43-bd2f-7c3055bfe589}
   PrintingTools, 1.2.5, true, {5e9999c2-ba1d-44b6-bcee-5b30ce37d3b0}
   Inverse SOGo Connector, 31.0.1, false, [email protected]
   Messaging Menu and Unity Launcher integration, 1.3.1, false, [email protected]

 Wichtige modifizierte Einstellungen

   Name: Wert

     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
     extensions.lastAppVersion: 52.7.0
     font.internaluseonly.changed: false
     font.size.variable.x-western: 12
     mail.openMessageBehavior.version: 1
     mailnews.database.global.datastore.id: 941b0feb-27db-4992-a611-1f842ed00ee
     media.gmp.storage.version.observed: 1
     media.webrtc.debug.aec_log_dir: /tmp
     media.webrtc.debug.log_file: /tmp/WebRTC.log
     network.cookie.cookieBehavior: 3
     network.cookie.lifetimePolicy: 2
     network.cookie.prefsMigrated: true
     network.IDN_show_punycode: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1523310776
     places.history.expiration.transient_current_max_pages: 81106
     plugin.importedState: true
     print.always_print_silent: false
     print.print_bgcolor: false
     print.print_bgimages: false
     print.print_colorspace: default
     print.print_downloadfonts: false
     print.print_duplex: 0
     print.print_evenpages: true
     print.print_in_color: true
     print.print_margin_bottom: 0.5
     print.print_margin_left: 0.5
     print.print_margin_right: 0.5
     print.print_margin_top: 0.5
     print.print_oddpages: true
     print.print_orientation: 0
     print.print_page_delay: 50
     print.print_paper_data: 0
     print.print_paper_height: 297,00
     print.print_paper_name: iso_a4
     print.print_paper_size_type: 1
     print.print_paper_size_unit: 1
     print.print_paper_width: 210,00
     print.print_plex_name: default
     print.print_resolution_name: default
     print.print_scaling: 1,00
     print.print_shrink_to_fit: false
     print.print_to_file: false
     print.print_unwriteable_margin_bottom: 56
     print.print_unwriteable_margin_left: 25
     print.print_unwriteable_margin_right: 25
     print.print_unwriteable_margin_top: 25

 Grafik

     GPU #1
     Beschreibung: Intel Open Source Technology Center -- Mesa DRI Intel(R) Bay Trail
     Herstellerkennung: Intel Open Source Technology Center
     Gerätekennung: Mesa DRI Intel(R) Bay Trail
     Treiber-Version: 3.0 Mesa 17.2.8

     Features
     WebGL-Renderer: Intel Open Source Technology Center -- Mesa DRI Intel(R) Bay Trail  -- 3.0 Mesa 17.2.8

     AzureCanvasBackend: skia
     AzureCanvasAccelerated: 0
     AzureFallbackCanvasBackend: none
     AzureContentBackend: skia
     CairoUseXRender: 0

 JavaScript

 Inkrementelle GC: 1

 Barrierefreiheit

   Aktiviert: 0
   Barrierefreiheit verhindern: 0

 Bibliotheken-Versionen

     Minimal vorausgesetzte Version
     Verwendete Version

     NSPR
     4.13.1
     4.13.1

     NSS
     3.28.6
     3.28.6

     NSS Util
     3.28.6
     3.28.6

     NSS SSL
     3.28.6
     3.28.6

     NSS S/MIME
     3.28.6
     3.28.6

Thanks for the info. I can see that you have created one mail account which has 6 identities. One will be the default and this will be seen as the mail account name in Folder Pane and in Account Settings. This is the one which is currently behaving as expected.

Check all others by doing the following.

• Right click on mail account in Folder Pane and select 'Settings'
• Click on 'Manage Identities' button

A small window opens and it will list all the various email addresses / identities. The following instructions need to be completed for each additional identity (not the top default one).

• Select the eg: second, identity in the list and click on 'Edit'

Another window opens...'configure the settings for this identity'

• click on 'Copies & Folders' tab
• select 'Place a copy in'
• select 'Other'
• select the default mail account name and 'Gesendet' folder
• click on 'OK'

Repeat the above actions on all the additional identities in the list.

• Finally click on 'OK' to save and close Account Settings.

Exit Thunderbird allow a little time for background processes to complete. Start Thunderbird

Additional helpful links http://kb.mozillazine.org/Mozilla_Suite_:_FAQs_:_Mail_Aliases https://support.mozilla.org/en-US/kb/using-identities https://support.mozilla.org/en-US/kb/configuration-options-accounts

Note: IMAP mail accounts see a remote view of server by subscribing to see server folders. All emails are stored on server. All imap folders synchronise with server folders. They are basically one and the same. So if you are sending from identity B, but want email stored in identity A sent folder. This is being put onto the server in identity A's sent folder. So the server for identity A email address must be allowed to receive and store identity B email address's sent emails. If this is an issue please read info below which may offer a better solution.

There is another solution which you could consider. Instead of using different identities for one mail account......... Create a separate mail account for each email address. So they act individually. This means each email address would be listed in the Folder Pane. Each would have their own set of folders. Then, in Thunderbird, you could use the 'Unified' view.

• 'Menu icon' > 'Folders' > 'Unified'

This would create one unified 'sent' folder. This would be a virtual folder showing everything sent from all mail accounts. You can also expand that folder to see individual 'Sent' folders. Image below shows an example of 'Unified' view in Folder Pane; I've expanded all folders to show the individual accounts within each Unified eg: 'Sent' folder.

Problem solved. Many thanks for the professional support.