Why is this happening?

[#1]

My email client is Thunderbird/
I belong to a Yahoo forum and receive messages via email to one of my mail boxes. However just recently a few messages have not gone to the allocated mail box but to Local Folders. I am curious what is causing the irregularity. Can anyone tell me from the attachments if it down to my set up or lies elsewhere?
I''ve attached a copy of the headers etc, amended for security.

[#2]

Bump! No one interested? Is thread  more appropriate to another section?

[#3]

mod:note
Please take a few minutes to read the link:rules on bumping 
I.e. don't be so blatant by posting the word `bump` 

[#4]

Hi Lucerne
I take it you have implemented a filtering rule to do this?

[#5]

@techguy. The set up is that  I have set up my profile on the Yahoo forum in question so that I can post to it via my mail box@mu user name, plus.com, and that all forum replies will be directed to me there also, On the mailbox in question there is no filter relating to the forum's address.
Do you think that is why some of the mail is going to Local Folders then? Why, if the address is the same in all cases, and the headers are identical some  emails should be directed to Local Folders confuses me. Surely a filter should not be required?
@chillypenguim

[#6]

I have to confess my ignorance of thunderbird ( use FF but use outlook as it is what I use at work so that I dont constantly have to switch between the two)
Is the sender address exactly the same on all mails? (meaning it does not change on the basis of which mailing node sent it as Yahoo have infrastructure here, there and everywhere.

[#7]

I see from the example emails that you attached that the "To" addresses seem the same. So thunderbird must have based its behaviour upon some other field in the message.

Quote from: lucerne

On the mailbox in question there is no filter relating to the forum's address.

Could there be a message filter on some part of the message other than the To address?
If you have no message filters setup, I was wondering if perhaps the cause could be part of the junk mail settings in Thunderbird. If Thunderbird suspects some of the messages to be spam then it could be following its Junk mail settings to move those messages. Try looking at Tools - Account Settings... - 'Yahoo mailbox' - Junk Settings. Check if the option for "move new messages to" - Other - is set to  "Inbox on Local Folders"?
For information, I use Thunderbird, but direct all Accounts to my Local Folders Inbox, rather than hold them separately. Then I use message filters if I want to recognise messages from a particular account.

[#8]

apologies, replied 'on the run' as just checked in before I headed out to work so missed your attachments, sorry for the dumb question.