This sounds like you are not using the correct smtp server to send the email.

Please check which smtp server 'Sender A' account is using. 'Tools ' > 'Account Settings' for mail account or right click on mail account in Folder pane and select 'Settings'. Bottom right - Outgoing Server (SMTP) - select the correct SMTP server for that mail account. See image below as aid. click on OK

If you are not sure about what to do please post the following information:

In Thunderbird

• Help > Troubleshooting Information
• click on 'copy text to clipboard'
• In the forum, in this question, right click in a 'Post a Reply' text box and select 'Paste'.

Tell me the number of the account which is 'Sender A'. Tell me the number of the account which recipient sees as sent from.

Thanks a lot! Regards, Michael

The spurious sender is account #2 This problem seems to affect all messages forward with any of the other accounts, but the most important one is account #4.

Ken West

 Application Basics

   Name: Thunderbird
   Version: 38.4.0
   User Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20151119061742
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts

   account2:
     INCOMING: account2, , (pop3) pophm.sympatico.ca:995, SSL, passwordCleartext
     OUTGOING: smtphm.sympatico.ca:587, alwaysSTARTTLS, 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
     OUTGOING: mail.tabtorontowest.ca:1025, plain, passwordCleartext, false