Please post this info:

In Thunderbird Help > Troubleshooting Information click on 'Copy text to clipboard' in this forum question, right click in a 'post a Reply' text box and select 'Paste'. you can edit/remove all info on fonts and printers but nothing else.

As requested, thanks for jumping in on this:

 Application Basics

   Name: Thunderbird
   Version: 38.5.0
   User Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20151221142744
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (none) Local Folders, plain, passwordCleartext

   account2:
     INCOMING: account2, , (pop3) pophm.sympatico.ca:995, SSL, passwordCleartext
     OUTGOING: smtphm.sympatico.ca:587, alwaysSTARTTLS, passwordCleartext, true

   account3:
     INCOMING: account3, , (pop3) mail.quidpro.ca:110, plain, passwordCleartext
     OUTGOING: mail.quidpro.ca:1025, plain, passwordCleartext, true

   account4:
     INCOMING: account4, , (pop3) mail.tabtorontowest.ca:110, plain, passwordCleartext
     OUTGOING: mail.tabtorontowest.ca:1025, plain, passwordCleartext, true
     OUTGOING: mail.tabtorontowest.ca:1025, plain, passwordCleartext, false

   account5:
     INCOMING: account5, , (pop3) mail.eol.ca:110, plain, passwordCleartext
     OUTGOING: smtp.primus.ca:465, SSL, passwordCleartext, true
     OUTGOING: smtp.primus.ca:465, SSL, passwordCleartext, false
     OUTGOING: smtp.primus.ca:465, SSL, passwordCleartext, false
     OUTGOING: smtp.primus.ca:465, SSL, passwordCleartext, false

   account6:
     INCOMING: account6, , (pop3) pop3.live.com:995, SSL, passwordCleartext
     OUTGOING: smtp-mail.outlook.com:587, alwaysSTARTTLS, passwordCleartext, true

   account7:
     INCOMING: account7, , (pop3) mail.quidpro.ca:110, plain, passwordCleartext
     OUTGOING: mail.quidpro.ca:1025, plain, passwordCleartext, true
     OUTGOING: mail.quidpro.ca:587, plain, passwordCleartext, false
     OUTGOING: mail.quidpro.ca:587, plain, passwordCleartext, false
     OUTGOING: mail.quidpro.ca:587, plain, passwordCleartext, false

 Crash Reports
   http://crash-stats.mozilla.com/report/index/bp-c2668934-dfdd-42db-918d-4d5162141219 (2014-12-19)
   http://crash-stats.mozilla.com/report/index/bp-6062de34-0dac-4c16-b11f-ffe6a2140612 (2014-06-12)

 Extensions
   Change quote and reply format, 0.8.6, true, {f6090211-2004-44d8-9090-be3c2adfd66f}
   Lightning, 4.0.5, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     accessibility.typeaheadfind.flashBar: 0
     browser.cache.disk.capacity: 819200
     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.zoom.full: true
     extensions.lastAppVersion: 38.5.0
     font.default.x-western: serif
     e.x-western: 14
     general.autoScroll: false
     gfx.blacklist.suggested-driver-version: 6.1400.1000.5218
     gfx.direct3d.last_used_feature_level_idx: 0
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: 29f689f3-d9ad-4fab-896c-b95f02ffad9
     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…
     network.cookie.cookieBehavior: 3
     network.cookie.lifetimePolicy: 2
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1452972015
     places.history.expiration.transient_current_max_pages: 104317
     plugin.importedState: true
     plugin.state.npgoogleupdate: 0

I notice you use POP mail accounts. Q: Does each mail account appear in the Folder Pane and have their own set of folders? OR are they set up as deferred accounts to use a Global Inbox (Local Folders), so you cannot see the mail accounts in the Folder Pane, but all emails go to Local Folders Inbox?

I can see that account numbers 4,5 and 7 have additional 'identities' Q: Confirm the extra identity email addresses have incoming mail going into the same Inbox as the default identity.

re : It seems to happen with one incoming account, Q: Which account number is experiencing the problem ?

A1: Only account 7 has its own folder: All others use the Local Folders.

A2: I cannot find anywhere in SETTINGS which determines where incoming mail goes. Incoming mail always goes to the INPUT folder. Everything else (except for 7) goes to is set to go to the respective folders in Local Folders: Sent, Archive, Drafts and Templates. Everything in 7 goes to the same folders in 7.

A3: *As far as I can tell*, the problem happens only when mail is returned/forwarded from account 4 tabtorontowest.ca . The incorrect account that is used for RETURN/FORWARD is account 2 sympatico.ca

Thanks for continuing to chase this.

Regarding the emails you receive into account 4: Are you the only recipient ? Did they come from someone sending a Mailing list or Rss subscription? Were you Bcc'd rather than TO 'd ?

If you select an email to see in Message Pane click on 'More' button and select 'View Source' Do you see 'Delivered -to:' or 'Envelope-to' headers which indicate that the message may have been sent using a list?

http://kb.mozillazine.org/Multiple_identities_per_e-mail_account Messages sent using mailing lists frequently don't specify what address the message was sent to in the To: or Cc: headers. It might be buried in one of the Received: headers, or in mailing list specific headers such Delivered-To: or Envelope-To: that Thunderbird has no knowledge of. In that case, Thunderbird doesn't know how to match the address the message was originally sent to and will typically select the default identity, which in your case is is account 2 sympatico.ca.

First make sure all identites are checked; 'Tools' > 'Options' > Advanced' > 'General' tab or 'Menu icon' > 'Options' > 'Options' > Advanced' > 'General' tab

• click on 'Config Editor' button - it will tell you to be careful :)
• In the top search type: self
• Look for this line: mailnews.reply_to_self_check_all_ident
• If Value = False, double click on the line to toggle from 'False' to True
• close window top right X
• click on OK to save changes in Options.

Did you need to change the value to True ?

Then check this setting: 'Tools' > 'Account Settings' > 'Server settings' for the pop mail account. or 'Menu icon' > 'Options' > 'Account Settings' > 'Server settings' for the pop mail account.

• click on 'Advanced' button
• select 'Include this server when getting new mail'
• click on OK
• click on OK to close Account Settings.

Was this checkbox already selected?

If you made any modifications please retest by selecting an email and click on 'Reply'. Is the 'FROM' email address correct?