Do you have separate smtp servers for each account, and are the accounts pointing to the correct servers?

https://support.mozilla.org/en-US/questions/1350593

The Gmail server information was set years ago according to their instructions and have worked for years. I've made no changes to that information for any of my accounts, and this is the only account that exhibits this behavior.

MORE INFORMATION: I'm discovering when I go to Account Settings>Outgoing Server>Edit SMTP server, if I edit the user name under Security and Authentification then the edited name is copied into several but not all of the other accounts.

That proves Sfhowes' assumption. That perception appears when each account does not have a unique server.

I don't understand how that applies. Gmail accounts all use the same outgoing server information with the exception of the user name.

You must have as many smtp servers as you have accounts: smtp1, smtp2, ...., all with the same settings - smtp.gmail.com, 465, SSL/TLS, OAuth2 - except different User Names (email address). Then, point account1 to smtp1, account2 to smtp2,..., as explained in the link in my first reply. Remove the entries from Saved Passwords in Settings, restart TB, enter the account password for each account in the OAuth2 window.

This doesn't explain how the (basically) reply-to address can be different than it should be WHEN NO CHANGES HAVE BEEN MADE TO SETTINGS.

Do you understand what I'm saying now?

Check your accounts and make sure they are set up as in the link in my first reply. Also, check you don't have any entries in the Reply-to Address fields in Account Settings.

Even if you haven't made any changes, doesn't mean the mail provider hasn't made changes, or it might have started to enforce certain rules.

You must have as many smtp servers as you have accounts: smtp1, smtp2, ...., all with the same settings - smtp.gmail.com, 465, SSL/TLS, OAuth2 - except different User Names (email address). Then, point account1 to smtp1, account2 to smtp2,..., as explained in the link in my first reply. Remove the entries from Saved Passwords in Settings, restart TB, enter the account password for each account in the OAuth2 window.

Thank you sir, for the detail explanation on this. I surmise this same (applicable smtp) premise applies to any created Identities in Thunderbird also(?).

sfhowes said

You must have as many smtp servers as you have accounts: smtp1, smtp2, ...., all with the same settings - smtp.gmail.com, 465, SSL/TLS, OAuth2 - except different User Names (email address). Then, point account1 to smtp1, account2 to smtp2,..., as explained in the link in my first reply. Remove the entries from Saved Passwords in Settings, restart TB, enter the account password for each account in the OAuth2 window.

Thank you sir, for the detail explanation on this. I surmise this same (applicable smtp) premise applies to any created Identities in Thunderbird also(?).

I think it depends on the provider. Some may allow sending from an identity on the same smtp as the parent account, but some, like gmail, might enforce strict matching between the sending account or identity, and the smtp User Name.

RB2124 said

sfhowes said

You must have as many smtp servers as you have accounts: smtp1, smtp2, ...., all with the same settings - smtp.gmail.com, 465, SSL/TLS, OAuth2 - except different User Names (email address). Then, point account1 to smtp1, account2 to smtp2,..., as explained in the link in my first reply. Remove the entries from Saved Passwords in Settings, restart TB, enter the account password for each account in the OAuth2 window.

Thank you sir, for the detail explanation on this. I surmise this same (applicable smtp) premise applies to any created Identities in Thunderbird also(?).

I think it depends on the provider. Some may allow sending from an identity on the same smtp as the parent account, but some, like gmail, might enforce strict matching between the sending account or identity, and the smtp User Name.

Thanks again, I tried this afternoon setting up several different IMAP accounts in Thunderbird for Gmail and like you said they seem to enforce strict matching email addresses for EACH account. I don't currently have any Identities in Thunderbird, but I remember in my Old Postbox version (which was based off Thunderbird code originally) it created a Default Group named 'All Accounts' if you created more than one account. It seemed like the Postbox Groups were the same premise as Thunderbird Identities. And I had two Spectrum accts in that old Postbox Group that all ran off the first account's smtp acct name data. In any case your replies to this thread helped me sort out all the details.