One additional clarification: This issue is not happening when I select one or more emails and then click on "Delete". This only happens when I am using the "Remove Duplicate Messages" add-on. This add-on has a setting as to what action to take when a duplicate email is to be deleted. I have selected "Move to Trash". So at least for the add-on, it is somehow deciding to define the Trash folder differently on my 2 computers. I have submitted a new issue to the owner of the add-on, but I have a feeling the addon is using some obscure Thunderbird setting to ascertain which Trash folder to use. Very strange. There is another item in the Community that made reference to a setting in the server portion of an account profile that is defined as "what to do when an email is deleted". I cannot find that setting on either of my computers.

Only Imap accounts will have the option to choose the 'Trash on the imap account'.

All Pop accounts will auto use the 'Deleted' folder for the Pop mail account. Most people will set up a Pop account to use it's own set of folders. So in Folder Pane you see the name/email address of the Pop account and it's own Inbox etc.

However, it is also possible to set up a Pop mail account as a deferred account - more commonly known as 'Global Inbox'. In this case, the Pop account is set up to use 'Local Folders' mail account. If set up correctly, you would not see the Pop account name folder in the Folder Pane and it would not use it's own set of folders.

This means it is possible to create eg: three Pop mail accounts, set them all up as 'Global Inbox' and therefore you would only see 'Local Folders' mail account in Folder Pane and it would receive all emails coming in for all three Pop accounts.

What is 'Local Folders' mail account? It's a special mail account created by default by Thunderbird. It is designed to be used for Pop accounts that are created as deferred accounts. It is also designed to be a place where any emails from any accounts can be stored. So, if you had an imap mail account and you had so many emails on the server that you were reaching a full quota, you could get copies of wanted emails stored in Local Folders and then delete the copy on the server thus creating more space on server. Local Folders mail account stores emails on your computer in your profile and no folder synchronises with any server.

Do you have any Pop account set up as a Global Inbox account ? If yes, then when you delete any emails from that account they would end up in Local Folders Trash folder.

How to find out if Pop account is set up as deferred account / Global Inbox:

• In 'Account Settings' > 'Server Settings' for the Pop account
• click on 'Advanced..' button

One of the following would be selected:

• If a normal Pop account - 'Inbox for this account'
• If set up as a deferred account - 'Inbox for different account' - 'Global Inbox (Local Folders)'

Toad-Hall: Thanks for your reply, but this is a normal POP account, based on your instructions. Based on my second post, I think the problem is related to the Remove Duplicates add-on because the Local Trash folder only gets used when I'm deleting Duplicate emails. But there's at least one other factor because it only happens on one of my PC's

Just a guess. As Toad alluded to. Under “Tools”/Account Settings, on the left select Server Settings for the account in question. Click on “Advanced”. That’s where the settings are for where the Messages go. Compare the settings on the two computers.

Joegons: Thanks for your reply, but you may have missed my response to Toad-hall where I verified that both of my computers do NOT have the Global inbox, they are standard POP accounts. So, there is no difference in the setup for the two computers. The only difference (and I don't see how it should cause this issue) is that the computer that is having the problem is running Thunderbird 91 while the other computer is running release 78. I'm still waiting for a response from the person who supports the Remove Duplicates add-on

Hi. On my setup - also POP from Gmail - the situation is that when I delete a message from the Inbox it goes to Delete folder under the mail account. When I delete a message from any of my (many) local folders thet end up in my local Deleted folder. Not a problem, just remember to clean up both Deleted folders. Regards Johannes Elkjær Madsen

re : the only difference (and I don't see how it should cause this issue) is that the computer that is having the problem is running Thunderbird 91 while the other computer is running release 78.

• Remove Duplicates 3.3 add-on
• Version 3.3 Released Sept. 16, 2020
• Works with Thunderbird 78.0 - 78.*
• This Add-on has been discontinued.

As updates are occuring in Thunderbird and this addon is not being maintained then you may expect to see bugs or unusual actions occuring. It should be working ok in the 78* version as it is designed for that version.

There was a change in where the folderpane data was stored, so maybe this effected the outcome and it is using Local Folders as a default when other account trash folders cannot be located.

re : the only difference (and I don't see how it should cause this issue) is that the computer that is having the problem is running Thunderbird 91 while the other computer is running release 78.

• Remove Duplicates 3.3 add-on
• Version 3.3 Released Sept. 16, 2020
• Works with Thunderbird 78.0 - 78.*
• This Add-on has been discontinued.

As updates are occuring in Thunderbird and this addon is not being maintained then you may expect to see bugs or unusual actions occuring. It should be working ok in the 78* version as it is designed for that version. There was a change in where the folderpane data was stored, so maybe this effected the outcome and it is using Local Folders as a default when other account trash folders cannot be located.

Unfortunately, there are/were 2 different add-ons with the same or very similar names. The one you mentioned isn't the one I'm using. Im using this one https://github.com/eyalroz/removedupes/

At the RemoveDupes link there is an image which has the option 'Move to' and you select where to move them to. As you say "the Local Trash folder only gets used when I'm deleting Duplicate emails." Check what folder you have selected in that 'Move to' option OR have you selected the 'Delete Permanently' option ?