The SMTP server should be selected based on the account shown in the from header in the compose window. So if the correct SMTP server is set in account settings then that is the one that should be used. The "default" should not come into it.

Check the identities pane as each identity setup in an account has it's own setting SMTP for that account.

I have done so. Both are correctly set in the account settings.

However they only work when I rotate the default setting in SMTP to that appropriate account selected. I make no other changes other than shift the default SMTP.

Hence my confusion.

When you say you have two yahoo accounts, do you mean you can see two mail accounts in the Folder Pane? So each mail account has only one identity.

Or do you mean you have one mail account with two identites ? Two yahoo email addresses going into one mail account?

Please post info: In Thunderbird

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Troubleshooting Information This page contains technical information that might be useful when you’re trying to solve a problem. If you are looking for answers to common questions about Thunderbird, check out our support website. Include account names (possibly identifying information) Application Basics Name Thunderbird Version 52.6.0 User Agent Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 Profile Folder (Local drive) Application Build ID 20180123185941 Enabled Plugins about:plugins Build Configuration about:buildconfig Memory Use about:memory Profiles about:profiles Mail and News Accounts ID Incoming server Outgoing servers Name Connection security Authentication method Name Connection security Authentication method Default? account1 (pop3) pop.mail.yahoo.com:995 SSL/TLS Normal password outbound.att.net:465 SSL/TLS Normal password true account2 (none) Local Folders None Normal password account3 (pop3) pop.mail.yahoo.com:995 SSL/TLS Normal password smtp.mail.att.net:587 SSL/TLS Normal password true Crash Reports Report ID Submitted

All Crash Reports Extensions Name Version Enabled ID Lightning 5.4.6 true {e2fda1a4-762b-4020-b5ad-a41df1933103} Important Modified Preferences Name Value accessibility.lastLoadDate 1522281948 browser.cache.disk.capacity 358400 browser.cache.disk.filesystem_reported 1 browser.zoom.full true extensions.lastAppVersion 52.6.0

mail.openMessageBehavior.version 1 mail.winsearch.firstRunDone true mailnews.database.global.datastore.id 6186c1a8-24de-410e-bf66-0ad212e1796 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… media.gmp.storage.version.observed 1 network.cookie.prefsMigrated true network.predictor.cleaned-up true places.database.lastMaintenance 1521739604 places.history.expiration.transient_current_max_pages 40983

security.sandbox.content.tempDirSuffix {ec65317a-7832-4ff7-8419-04af139f3dc1} Graphics GPU #1 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 23.20.16.4849 Driver Date 10-27-2017 GPU #2 Description NVIDIA GeForce 940MX Vendor ID 0x10de Device ID 0x134d RAM 2048 Drivers C:\WINDOWS\System32\DriverStore\FileRepository\nvhm.inf_amd64_c8a41364c1b3daa8\nvldumdx.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nvhm.inf_amd64_c8a41364c1b3daa8\nvldumdx.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nvhm.inf_amd64_c8a41364c1b3daa8\nvldumdx.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nvhm.inf_amd64_c8a41364c1b3daa8\nvldumdx.dll C:\WINDOWS\System32\DriverStore\FileRepository\nvhm.inf_amd64_c8a41364c1b3daa8\nvldumd.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nvhm.inf_amd64_c8a41364c1b3daa8\nvldumd.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nvhm.inf_amd64_c8a41364c1b3daa8\nvldumd.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nvhm.inf_amd64_c8a41364c1b3daa8\nvldumd.dll Driver Version 23.21.13.8873 Driver Date 12-18-2017 Features Direct2D false DirectWrite true (10.0.16299.15) WebGL Renderer Google Inc. -- ANGLE (Intel(R) HD Graphics 620 Direct3D9Ex vs_3_0 ps_3_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 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

Noted : both accounts use POP.

I can see you are using the yahoo incoming server for both, but still using the original att smtp for outgoing.

Account 1 is using 'outbound.att.net' Account 3 is using 'smtp.mail.att.net'

According to the information located on att website, Account 3 is not using the correct smtp server. smtp.mail.att.net is for imap mail accounts and you are using pop. https://www.att.com/esupport/article.html#!/dsl-high-speed/KM1010523

Based on this, I believe you are having trouble using the 'smtp.mail.att.net' server settings, so suggest you edit the smtp settings.

You should then have two outgoing servers using same server settings, but each will use different User Name and different passwords. When selecting an smtp server for the account to use, it may be difficult as the two look the same. So, whilst editing the smtp server settings, use the 'Description' field to help you identify between the two, you can type anything in the 'Description' field eg: Personal, Work or even the email address itself.

Then make sure each account is using correct smtp server.

Whilst locating info, I came across the following information which may be useful; assuming you have not come across this yet. https://www.att.com/esupport/article.html#!/email-support/KM1240462

POP: Create a secure mail key and enter it in place of your AT&T password in your existing email application.

Even with the same SMTP settings of outbound.att.net I run into the same problem.

If I change the default account to the other SMTP, then I can send. I can only send from one email and that email is the one with the default SMTP.