Please provide more info. You mentioned moving a message from a unified folder and later deleting from a unified folder. How can it be moved and yet still be there? I think I'm missing something here. Thanks.

Thanks for responding. I move the email from unified to another folder, but then it bounces back to unified AND also appears in the other folder. If I delete the email in unified it also disappears from the folder.

Okay, maybe some more details will help. Is this an IMAP account? Is this the inbox? Is the receiving folder also a unified folder? Is the receiving folder also a folder within the same account? Is this a Gmail account? If so, what is the name of the sending folder and receiving folder?

This is an imap account. It is the inbox. Not sure I understand the rest of the questions - because I'm not very techy. So, I'll answer the best I can. The unified folder contains 3 email accounts. One is gmail. The other two are accounts with my own domain email address. The specific folders I want to move the emails into are within the gmail account. The emails might come to me with a gmail address or my domain address. Sending folder / receiving folder ? Not sure I understand this but here goes - Sending folder - unified. Receiving folder - e.g. holiday 25. Never had a problem before.

Ok, I won't pursue further, but I had anticipated this was a Gmail account. Unlike other email providers, Gmail actually uses just one folder for everything, assigning labels to differentiate what other email providers call folders. Thunderbird (and other email clients) use folders for consistency, which means that Thunderbird must duplicate messages when a move is made. There is a special folder for Gmail, AllMail, which points to all messages, regardless of location. The unified view is not a part of this; that unified view is a display option that Thunderbird constructs to give the impression of one inbox, but that is just an illusion. You may be able to isolate this issue better by selecting View>folders and tick 'all folders' or rightclick the unified folder icon to expand the view to show individual accounts. My 'rambling' here is not meant to solve the issue, but to assist you in viewing the folders with more insight. Let me know how it goes.

Thank you David and with your answer I think you may have helped me go forward with "all folders" vs "unified folders". I will investigate.