Not exactly sure what you're asking for. What's the problem with adding your Postfix server as a new SMTP server?

Hello Christ1 thanks for your question. Perhaps I'm NOT sure what I'm asking, but I'll try to explain it. I define users to thunderbird (which have already been defined to my linux server) and "point" them all to the same outgoing smtp server. However I get the following message: Sending of the message failed. The message could not be sent because connecting to Outgoing server (SMTP) sendmail.ids4zos.net failed. The server may be unavailable or is refusing Outgoing server (SMTP) connections. Please verify that your Outgoing server (SMTP) settings are correct and try again.

looking in the installed directories for thunderbird, I don't find any documentation on configuration nor any log files. I tried running a tcpdump on the sendmail.ids4zos.net ip address and tried to capture all port 25 packets, but there are none. I can ping sendmail.ids4zos.net, and I can do an nslookup and find the name. I've also defined sendmail.ids4zos.net to DNS with an MX record. There are no entries in the maillog either, so I don't know how to troubleshoot this problem. Any ideas and/or suggestions would be most appreciated. Cheers Guy

Can you post your Troubleshooting Information? At the top right of the Thunderbird window, click the menu button , then select Help, and then Troubleshooting Information.

Is sendmail.ids4zos.net a local server on the same machine as Thunderbird, or a machine in the same LAN? Or is it a machine reachable via Internet?

Hi Christ1, thanks for the reference to the troubleshooting information. I've attached the file and a screenshot of the error information. It contains a LOT or errors regarding chrome which I don't understand.

sendmail.ids4zos.net is a local server on the same machine as TBird.

Below is a copy of the Thunderbird Info

   Application Basics
   Name: Thunderbird  Version: 38.3.0
   User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0
   Profile Directory: /home/grich/.thunderbird/52c3w4d5.default    (Local drive)
   Application Build ID: 20150929044857
   Enabled Plugins: about:plugins  Build Configuration: about:buildconfig    Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, GRich@ids4zos.net, (imap) recvmail.ids4zos.net:143, plain, passwordCleartext
     OUTGOING: sendmail.ids4zos.net:25, plain, passwordCleartext, true
   account2:
     INCOMING: account2, Local Folders, (none) Local Folders, plain, passwordCleartext
 Extensions
   Lightning, 4.0.3, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences
   Name: Value
     browser.cache.disk.capacity: 358400   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: 38.3.0    mail.openMessageBehavior.version: 1
     mailnews.database.global.datastore.id: eefc8a5f-7b9e-4a1c-aad6-58136a821a7
     network.cookie.prefsMigrated: true  plugin.importedState: true

 Graphics
     Adapter Description: Intel Open Source Technology Center -- Mesa DRI Intel(R) Ivybridge Desktop
     Vendor ID: Intel Open Source Technology Center
     Device ID: Mesa DRI Intel(R) Ivybridge Desktop
     Driver Version: 3.0 Mesa 10.2.7
     WebGL Renderer: false  GPU Accelerated Windows: 0
     AzureCanvasBackend: cairo      AzureSkiaAccelerated: 0   AzureFallbackCanvasBackend: none
     AzureContentBackend: cairo

 JavaScript
 Incremental GC: 1   Accessibility     Activated: 0     Prevent Accessibility: 0

 Library Versions
     Expected minimum version
     Version in use

     NSPR             NSS				NSS Util
     4.10.8	   3.19.1 Basic ECC		3.19.1		
     4.10.8	   3.19.1 Extended ECC		 3.19.1

     NSS SSL
     3.19.1 Basic ECC
     3.19.1 Extended ECC

     NSS S/MIME
     3.19.1 Basic ECC
     3.19.1 Extended ECC

Name resolution doesn't seem to be the problem. Did you set up your firewall to allow connections to TCP port 25? Not sure what distribution you're running. Some distributions ship with an additional layer of security using software like AppArmor or Selinux. Those require an additional rule set what applications are allowed to do (or not allowed). Check the message log for your machine, the Postfix log, and the firewall log. This should give a clue. I don't think the problem is Thunderbird here.

I'm using RHEL 7.0. the sendmail server is "behind" the firewall and port 25 traffic is allowed. Redhat's version of Selinux is very good at alerting via pop-up message whenever a rule violation is detected, and it also gives a solution. However I will check the firewall log. Does TBird attempt a direct connection with the send mail server ? or does it "talk" to an MDA ?

Guy