First thing is you have to check that A is sending on the A smtp and B is sending on the B smtp. With gmail, if the From account is B, but B is sending on A's smtp, then gmail will switch accounts so that recipients see the message as coming from A.

Tools/Account Settings, select an account in the left pane, then look at Outgoing Server (SMTP) in the lower right pane. A is probably sending on the Default smtp which applies A's password and User Name. If you select B in the left pane, its smtp should not be Default, i.e. smtp for A, but an smtp with B's password and User Name.

The other issue, when a message is sent to both A and B: when you reply to a message, TB sets the From address to be the account that includes the folder with the message. That is, if the message is in A's Inbox, a reply will be From A, if it's in B's Inbox (or subfolder) the reply will be from B. Confusion may ensue if the smtp servers aren't configured as in the previous paragraph.

There are add-ons to manage multiple accounts and identities, such as:

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

Thanks very much for your reply!

I'm using my ISP SMTP same credentials for all email accounts (there are actually many) and all these different email accounts end up in a single inbox.

Do you mean there's no way of being able to tell from either looking at the email, or pretending to reply to that email, whether the email was sent to either A or B and that the only way for Thunderbird to figure this out is by dividing my inbox into seperate inboxes for each email address?

Thunderbird behaves fine when the email is sent to just one of my email addresses. It replies from the correct address.

It doesn't behave fine when two of my email addresses are on the same distribution list (it tries to send both emails from my default A, unless I intervene)

It doesn't behave fine when one of my addresses is on a BCC email, I have no idea which address received it and trying to reply just defaults to A unless I intervene.

The gmail part is a red herring, sorry. This behavior happens for all my email addresses whether outlook, hotmail, gmail, anothermail.

Any further help is much appreciated!

re :I'm using my ISP SMTP same credentials for all email accounts Ideally, each mail account should have it's own SMTP and send through it's own server as explained by sfhowes.

re: all these different email accounts end up in a single inbox. Can you offer a bit more information on the setup. Do you mean: A: all the accounts have been created as POP mail accounts and they are set up as deferred accounts that use a Global Inbox and is that Global Inbox called 'Local Folders' Inbox? Therefore you do not see individual mail accounts in the Folder Pane, only 'Local Folders' which contains all emails from all accounts and all incoming mail is contained in the one 'Local Folders' Inbox ?

Or B: similar to above, where you can see mail account of Account A and Local Folders mail account in Folder Pane, but have set the Global Inbox to be Account A, so all other POP mail accounts are not visible in Folder Pane?

OR do you have all emails from all email address being auto forwarded from servers into the ISP Account A email address server and then you have only to download everything into the one email address (Account A) mail account ?

Hello Toad-Hall,

This!:

A: all the accounts have been created as POP mail accounts and they are set up as deferred accounts that use a Global Inbox and is that Global Inbox called 'Local Folders' Inbox? Therefore you do not see individual mail accounts in the Folder Pane, only 'Local Folders' which contains all emails from all accounts and all incoming mail is contained in the one 'Local Folders' Inbox ?

I set it up exactly as my Outlook Express was set up. I'm happy with Thunderbird generally, apart from this issue - this issue is not present in Outlook Express, it figures out which email address received the email and defaults to that automatically when I begin to reply.

re :I'm using my ISP SMTP same credentials for all email accounts Ideally, each mail account should have it's own SMTP and send through it's own server as explained by sfhowes.

This I don't understand (please bear with me).

The SMTP credentials would be the same for ALL outlook.com or ALL gmail.com addresses - i.e. server for all outlook and server all for gmail with no care given by either server what respective address was sending through it.

So I have simply replaced both with a single ISP SMTP server.

That's whats worked fine in Outlook Express for over 15 years and works fine in Thunderbird.

Because I'm obviously inexperienced with Thunderbird, it may be a thing to have to create individual inboxes for the mail client to be able to handle things properly.

That's not really an option for me. All I need is for Thunderbird to know what email address to send a reply from - i.e. the same address that received the email!

re :The SMTP credentials would be the same for ALL outlook.com or ALL gmail.com addresses - i.e. server for all outlook and server all for gmail with no care given by either server what respective address was sending through it.

Not quite correct... ...with no care given by either server what respective address was sending through it. You should make sure that the correct email address is going to the correct accounts smtp.

Gmail has its own smtp server for gmail email addresses.

The server settings are basically the same, but the Username' will be the accounts respective full email address and the password would be the one you use to access that particular gmail email address webmail account via a browser. So make sure that gmail account A is actually using smtp A with same email address in Username.

In Account Settings Select 'Outgoing Server SMTP' - bottom of left Pane.

Perhaps you still have the gmail smtp servers and just need to check them and reassign to the correct account. Select gmail smtp in list and click on 'Edit' OR click on 'Add' to create one.

Description: Type something useful to help you differentiate between the gmail accounts eg: email address or Personal or Business Gmail etc

Server name: smtp.gmail.com

Then use either Port: 465 Connection Security:SSL/TLS

OR Port: 587 Connection Security: STARTTLS

Authentication method: Password UserName: full gmail email address

click on OK

Do the same for the other gmail email address.

Now make sure that the outgoing server where 'username' = email address is being used as the outgoing server for the mail account with same email address as username. Select the gmail mail account bottom right where it says Outgoing server (SMTP) click on the drop down to locate the correct gmail smtp server - note this is where the Description field is really useful in locating correct smtp.

Then do the same for the other gmail account.

Then go through same process to set up the hotmail accounts. Hotmail accounts will also have their own smtp server. Some info: https://support.mozilla.org/en-US/kb/thunderbird-and-hotmail#w_configuring-the-outgoing-server

When finished click on OK

Restart Thunderbird.

When you first try to send using each email address, you will be asked for correct password for that gmail or hotmail etc email address. select checkbox to tell Password manager to remember password click on OK.

Having all accounts send through the same smtp server of your ISP may have been feasible 3 or 4 years ago, but that setup is obsolete for today's security standards and authentication requirements. Not to mention the ability to send mail over any network. The account setup process in the next version of TB, currently in beta, reflects this by explicitly forcing the user to initially specify the outgoing server at the same time as the incoming server.

Also, for gmail IMAP accounts, it's recommended to use OAuth2 authentication, as this avoids issues with 2-step verification, allowing 'less-secure apps' and app passwords. OAuth2 isn't currently supported in TB for POP accounts, but there are other settings that may need attention if you download gmail POP from multiple computers.

http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite

First of all thanks to you both for your help towards what must seem a complete luddite.

I think part of the issue is when I installed Thunderbird it automatically migrated everything from Outlook Express, part of which was all my mail accounts and placed them under "Local Folders" with a global inbox.

So, after carrying out various tests according to your info I ended up deleting one of my @gmail accounts from account settings and recreated it again. This then created a separate individual inbox for that account.

I went through the process again of sending a mail and BCCing the gmail and an outlook address. The gmail popped into the gmail inbox and the outlook popped into the global inbox.

I tried to reply to the @gmail email and success, it automatically chose the correct address. Even when I moved this email from it's own individual inbox to the global inbox it still behaved properly when I tried to reply.

The @outlook email (in the global inbox) still behaved in the same incorrect way, that is Thunderbird tried to reply from my default 'A' email.

So I think the normal behavior of Thunderbird is to have each individual email inbox separate and it works properly.

Regarding the SMTP situation, I tested using my ISP SMTP and gmail SMTP. There was no difference to me, everything behaved as it should regardless. So, rightly or wrongly, I may just continue to use just the single ISP SMTP server and change if there becomes an issue.

One further question, which I hope you don't mind - it is a little annoying having to check a list of 10 new inboxes and manually move messages to the global one. Can a rule be created in Thunderbird to move new messages from each inbox into the global one under "Local Folders"?

I confess I really don't see the point of Global Inbox. The Unified view, from View/Folders/Unified, which works for POP and IMAP accounts, shows both the single Inbox with all messages, and inboxes for each account. I also find that when I click Reply to a message in the Unified Inbox, the From account is the same as the To account - not the Default account.