re :The smtp settings are the same as the other four working accounts.

All four accounts would use identical server settings with a notable exception - the user name and the password would be different.

In the Account Settings > Outgoing Server (SMTP) Do not access via Account Settings and select name of account and use 'Edit SMTP_server' button. It is too easy to edit the wrong account if the selection is incorrect.

Check: You should have four smtp listed. Select each in turn - Select 'Edit' Description - enter something useful to help identify the account eg: full email address (the same full email address that is in the 'User Name') or something like 'Business' or 'Personal' etc Then ok Check all the others.

Then make sure the actual account is using that correct smtp server. Select mail account name in left pane - on right side at bottom - Outgoing Server SMTP - use drop down to select correct smtp for that email address. Check all accounts are using the correct smtp server.

Exit Thunderbird and restart.

G'Day Toad-Hall Thankyou for your reply but I had done all of those steps several times over two days of troubleshooting. At around 13:40 (Brisbane time) the fault just cleared. I was in the middle of testing the account from a variety of devices, when I noticed that it appeared to be working when it wasn't previously. On going back to my "normal" Win10 desktop PC I confirmed that I could now send from the account and it has been fine ever since. I can only summise that something has changed at the ISP end.

I will do a Mozbackup while it is working.

Cheers, gaz

re :I will do a Mozbackup while it is working.

Strongly advise you do not use 'MozBackup' as it has not been updated in a decade and even the author advises it has bugs and use at own risk.

It is much easier to just exit Thunderbird and access Roaming folder and make a copy of the Thunderbird folder.

• Menu app icon > Help > More Troubleshooting Information
• Under 'Application Basics' - 'Profile folder' - click on 'Open Folder'

A new window opens showing the content of the 'profile name' folder.

• Exit Thunderbird now.

• Go up the directory 3 levels to the 'Roaming' folder and copy the 'Thunderbird' folder to an external drive.

The issue is that bigplod is still working in the 1990s, perhaps they need another cash injection from the government to improve their security and bring it into the 231st century.

Some years ago there was a serious vulnerability identified called logjam at this time to mitigate the issue the encryption keys were increased in size to 2048bits. I did a test on the telstra server an that resulted can be seen here. Under the heading DIFFIE-HELLMAN PARAMETER WEAK the key is shown to be half of what is required to be secure.

The settings below for Telstra/Bigpond are still working for me to send mail. They have been unchanged for probably 10 years. I did a test send yesterday as I have not sent anything on the Telstra service in years and mail was sent and received in the account it was addresses to. So I suggest you try those.

Note that there is no encryption,and that is your issue, but given I use a wired connection from my desktop to Telstras network I don't think that really represents any risk. Neither my password nor my email are unencrypted outside of the Telstra network. Unlike say sending via gmail where the whole thing leaves the ISP network and ventures out into the public internet

@ Toad-Hall Thanks for the heads up on Mozbakup - I will take your advice.

@ Matt Thanks for the detailed explanation and test results. They are very enlightening and could well explain my issue. I do not understand the import of many of the items but a C rating and non compliance since 2018 is enough to make me consider sending a complaint to Telstra. They really need to improve their game in many areas.