re :When I am reading mails in ONE inbox, and press the "Write"-button to create a new outgoing email, Thunderbird assumes that I want to send from that address.

that is correct.

You could add the 'default' mail account email address as an identity to the other mail accounts. Right click on mail account name in Folder Pane and select 'SEttings' click on 'Manage Identities' click on 'Add' Add the default name, default email address and select the default outgoing server. (Basically forcing all new Write messages to compose as if using the main default account) select the added identity email address as click on 'Set as default'. click on 'Close' click on OK

This means when click on Write, your FROM will be the default identity. Reply should still use the FROM that the account was received in.

There are a couple of add-ons for managing multiple identities:

https://addons.thunderbird.net/en-us/thunderbird/addon/identity-chooser/

to control which account is the sending identity, and

https://addons.thunderbird.net/en-us/thunderbird/addon/folder-account/

to set the sending identity or account on a per folder basis. Both add-ons have versions for TB 60 and 68.

I have the OPPOSITE PROBLEM. When I specify that I want to send an email from a specific one of my email accounts, typically NOT the one that Thunderbird lists "first", the software *instead* sends it from that WRONGFULLY PRESUMED "primary" email address. Extremely noxious result is that the people that I'm writing too are invariably filed and processed and see me in relation to a totally different email address than the one that Thunderbird is wrongfully imposing on my outgoings. I'm accurately specifying my sending address when I write the emails but Thunderbird is contradicting me and replacing what I DEMANDED with some botched version of its own wrongful choice. HOW can I get Thunderbird to do what *I* am telling it to do?

bobgru said

I have the OPPOSITE PROBLEM. When I specify that I want to send an email from a specific one of my email accounts, typically NOT the one that Thunderbird lists "first", the software *instead* sends it from that WRONGFULLY PRESUMED "primary" email address. Extremely noxious result is that the people that I'm writing too are invariably filed and processed and see me in relation to a totally different email address than the one that Thunderbird is wrongfully imposing on my outgoings. I'm accurately specifying my sending address when I write the emails but Thunderbird is contradicting me and replacing what I DEMANDED with some botched version of its own wrongful choice. HOW can I get Thunderbird to do what *I* am telling it to do?

Open Tools/Account Settings, select an account in the left pane, then click Edit SMTP server in the lower right pane. Is the User Name of the smtp the same as the selected account? If it isn't, recipients may see the sender as the smtp's User Name, not the From: account. Gmail is an example that will automatically replace the From: account with the gmail smtp, unless the account is added to gmail, as an authorized sender. Check that each account is sending on an smtp with matching User Name, not a Default smtp for all accounts.

Each of the SMTP settings is accurate for its related account. The problem isn't happening when I'm in Thunderbird itself at the time that I'm writing an email. When I'm within my intended account at the time of writing, it always uses *that* account's proper sending address and signature lines. When the problem arises is when I'm on somebody else's web site invoking an incidence of Thunderbird for sending an email to the owners of the web site. I specify which of my email accounts I *want* to send the email from, the proper sig lines are displayed and, for that matter, the proper sending address is *displayed* on what I see but then Thunderbird is over riding what I *saw* and *intended* by replacing the sending address *and* the sig lines with what is appropriate only for what it considers to be my "primary" account (where I get the vast majority of my incomings and virtually never send any outgoings).

If you click a mailto link on a web page, a new message is generated in TB, and the From address is always the Default account. But you are free at that point to change the From before sending. If, as you say, all your accounts are matched with the correct smtp, I can't explain why the messages are sent from the Default. There have been issues with accounts not being checked for new mail at startup, due to damaged Default account settings. Perhaps if you change the default account, from Account Actions in Account Settings, it will change the outcome.

@sfhowes I've got another thing recently developed which resembles what you were talking about "accounts not being checked for new mail at startup, due to damaged Default account settings". Just had that momentarily when I fired up Thunderbird *this* time. But I'm also getting strange error messages when I try to send anything originating from a web contact invoked incidence of Thunderbird which refuses to send and says I have to "check account settings". When I do a "get messages" for the specific account it then allows sending. As for "Perhaps if you change the default account, from Account Actions in Account Settings, it will change the outcome", I see that in fact the TBird itself styled "primary" account is in fact identified as the "Default" account in those account Settings and that when I click on some other email address it is willing to change *that* to being the "default" account. My problem is that *I* don't really want a "default" account at all but really want to be able to select whichever of my plethora that I want used for any specific email. Inspecting this situation further it is "conceivable" that the set of original falsely addressed outgoings were my own fault (that I "thought" I changed to the proper sending address but actually didn't) but I can't be sure of even that with the other issues that have surfaced.

I think it was true in the past that some mail services required one to check for new mail before sending could occur, as a means of authenticating the user. But nowadays, any reputable service requires authentication on sending - very few operate smtp on port 25 with no authentication.

As for the default account, I don't think it's wise to not have one, for the reasons stated above. If your accounts are set up properly, having a default account shouldn't interfere with normal operations.

In essence, trying to make use of a "mailto" address while visiting a web site is NOT a "normal" operation. Virtually, well nearly, all of my outgoings are generated either in response to a specific incoming (such that I'm always in the appropriate email account when replying) or from me to *known* others from the specific email account where I file all of our communications. This was simply a triplicate wierdness where I kept finding my outgoings in that "not a good choice but there really isn't any better single choice" default account. For some reason the wierdness had never happened before and it *is* possible that I was so focused on why I was trying to contact the web site operators that I didn't pay adequate attention to my own "From" line and its related sig lines. Thanks for your comments on my strange situation.