Imap accounts show subscribed folders which synchronise with server. So in effect, imap accounts are virtual copy of what is on server, they can only show what is on server. Anything set via webmail, Thunderbird imap account, another imap account on an other computer or phone etc accessing the same email address/account will effect the original copy on server. As folders synchronise with server, you cannot rely on the copies in Thunderbird as a separate copy. They are in effect one and the same. If the server has already marked as read before Thunderbird synchronises then Thunderbird can only show them as marked as read. If Thunderbird were to run a Message Filter which basically resets all downloaded emails to Unread, then this will also occur on the server which may not be the desired result.

You have to be more inventive to work with the system. Such as: Create a TAG called 'Unread' or it could be called by your name and give it a colour that stands out. As new emails are downloaded, you could have a 'Message Filter' that TAGs all new mail with your 'Unread/Name' tag. Note some servers will still show tags, some only show Thunderbird default tags and not newly created ones. If the tags are visible on server, then you can inform others accessing the same account know, that particular tag is set by you and should only be removed by you. As you read emails, you remove the tag.

Another idea: you could create folders in 'Local Folders' mail account and run Message Filter which puts a 'Copy' of all new emails in 'Local Folders' mail account 'Inbox'. Do not set an unread flag in the filter because it will effect copy on server. Then assume copied emails are unread until you move them to another 'Local Folders' folder. Thus keeping the 'Local Folders' Inbox for only assumed unread emails.

Hi, thank you for your reply, it provided me with a solution. I simply made a filter that would set the status as unread when getting new mail sent to my email address. Problem solved!

Cheers Tor Holm