I would think that you could simply set your outgoing server (SMTP) settings the same for all of your accounts or simply set the default server.

Thank you for replying. I went through every setting in Thunderbird and there is no where I can set the default email program. I have 4 email accounts that I used for various purposes and the 1st email account that I set up became my default email account when sending. Each email account is separate from each other. They all have their own default settings. In other words, each account becomes its own default account. If I replied to someone's message. It automatically uses that account to send. Before Microsoft stop allowing those of us who have Windows office. It was easy to do to set the default account.

I appreciate your help, but no such luck. Have a good day!

The default is set in outgoing server (SMTP) in account settings.

However there are a large list of caveats attached to the default. If you are in an account (in the all folder view), and the identity that received the email will be the default when you reply. If you have multiple identities configured for a single account then all of them get first shot before the "default" is selected.

The default is not a "use this unless I say different" type of proposition. You might want to look at this addon. Perhaps it will meet your needs. https://addons.thunderbird.net/en-US/thunderbird/addon/identity-chooser/?src=cb-dl-users