I would guess something changed at ATT. Is there some reason you can not use the correct SMTP server to send the mail instead of the ATT one?

ATT says nothing has changed. I just prefer to have my emails only show my forwarding address (rather than showing the ATT address and then including a "reply to" line with the forwarding address. I was able to do this until Tuesday, and then I started getting error messages from Thunderbird.

Clearly something has changed. Thanks ATT for your usual high standard of gold level support.

But you did not answer the question I asked. What is the reason you are using ATT SMTP server instead of the correct one for the relevant mail account?

I just prefer to have my emails only show my forwarding address (rather than showing the ATT address and then including a "reply to" line with the forwarding address.

That is not a reason to be using ATT SMTP server.

Thank you for your help. I apologize for not understanding your question previously.

Thunderbird Account Settings show the correct outbound server (outbound.att.net), and the settings for that outbound server show my correct ATT user name. Several years ago I changed the address shown in the Email Address box of Account Settings from my ATT address to the forwarding address. (This did not change the outbound server or my user name in Account Settings.) Everything worked fine, I was able to send emails and they all showed that they came from the forwarding address. Earlier this week I started receiving error messages every time I sent an email. The only way I found to fix this was to remove my forwarding address from the Email Address box in Account Settings and insert my ATT address. That allowed me to send emails, but they show they are coming from the ATT address (adding my forwarding address as a reply to address).

Is the incoming address @ATT? if it is not it is not the correct server to use.

lets cut to the chase, please post the troubleshooting information to your post (or at least the mail and news accounts part of it. The rest is not really relevant to this discussion.

To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.

Here it is:

 Application Basics

   Name: Thunderbird
   Version: 52.4.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20171004092449
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account9:
     INCOMING: account9, , (pop3) inbound.att.net:995, SSL, passwordCleartext
     OUTGOING: , outbound.att.net:465, SSL, passwordCleartext, true

 Crash Reports

 Extensions
   Lightning, 5.4.4, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

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     places.database.lastMaintenance: 1511641946
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 Graphics

     GPU #1
     Description: DisplayLink USB Device
     Vendor ID: 0x000b
     Device ID: 0x000b
     RAM: Unknown
     Drivers: Unknown

     GPU #2
     Description: Intel(R) HD Graphics 620
     Vendor ID: 0x8086
     Device ID: 0x5916
     RAM: Unknown
     Drivers: igdumdim64 igd10iumd64 igd10iumd64 igd12umd64 igdumdim32 igd10iumd32 igd10iumd32 igd12umd32
     Driver Version: 21.20.16.4574
     Driver Date: 12-23-2016

     Features
     Direct2D: Blocked for your graphics card because of unresolved driver issues.
     DirectWrite: true (10.0.15063.674)
     WebGL Renderer: Google Inc. -- ANGLE (Microsoft Basic Render Driver Direct3D11 vs_5_0 ps_5_0) -- OpenGL ES 2.0 (ANGLE 2.1.0.2a250c8a0e15)

     AzureCanvasBackend: skia
     AzureCanvasAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: skia

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

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     4.13.1
     4.13.1

     NSS
     3.28.6
     3.28.6

     NSS Util
     3.28.6
     3.28.6

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     NSS S/MIME
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