Well the message is clear. the Bin folder that your trash is being redirected to is missing. Most unusual to have a folder called Bin, it must have been some sort of ntworld special. Usually trash folders are called trash or deleted. Back in the 1980s Apple sued anyone that used the term Bin, apparently they though that had some sort of intellectual property on the name.

So right click the account in the folder pane, select subscribe. This will show a listing of all folders on the IMAP account on the server. Note the name that looks like it is meant to hold deleted mail.

Right click the folder again and select settings. In Server settings for the account select the folder you identified in the last step as the one deleted mail is to be copied to.

Mail is marked as read when it is displayed in the reading pane at the bottom of the screen.

Thanks for your reply. I have tried just deleting the e-mails today and they are moved to a deleted folder. I didn't try your solution. I don't know what went off and why it is now working on its own. I will bear your solution in mind in case it happens again. Again thanks for your reply.