Manually set IMAP Deleted folder in Mozilla Thunderbird
Mozilla Thunderbird's Trash folder name does not match the Trash or deleted folder name on your IMAP server.
Cause of the Problem:
By default Mozilla Thunderbird will always try to use "Trash" as the deleted items folder. However not all IMAP servers use the folder name Trash for their deleted items. This causes another folder to appear in your account in Thunderbird. Deleting emails in Thunderbird still sends them to the Trash folder, and not the appropriate folder.
There is a way to manually configure Thunderbird to know what the actual name of your Trash or deleted folder is.
Go to the Tools menu, and choose Options.
Go to the "Advanced" section and the "General" tab. Hit the "Config Editor" button located next to "Advanced Configuration".
Now you need to do some research hereto find which server you need to modify. Use the filter input box at the top, type in mail.server.server and you will see the list change and narrow down. This is a list of preferences and their associated values. One of these will be mail.server.serverX.name, where X is a number and the value is the name of the IMAP account you want to adjust the Trash folder for. Remember X because you need it to create the preference in the next step.
You can Right-click in a empty space in the list and select New->String from the shortcut menu.
You will be prompted for the name of your new preference. Put in mail.server.serverX.trash_folder_name, where X is the number you took note of from above.
After this a new prompt will ask you for the value of the preference. Put in the name of the folder as it is on the server. If it is a nested folder be sure to have the full path to the folder.
Close any remaining dialog boxes and restart Thunderbird for the changes to take effect.
Create a test message and delete it to verify that it is being sent to the correct folder on the server, and in Thunderbird.