SBS2011 - Exchange 2010

Hey everyone,
I have the following problem with a customer using a SBS2011, means Exchange 2010 latest patch level:
there was a mail-enabled public mail folder. For several months it worked fine that one certain mail address was sorted into that certain public folder.
Since a few weeks some other mails dedicated to regular users are sorted into that folder too, but not always. sometimes they are directed into the folder sometimes they are sorted to the user they belong to.
Now I disabled the mail function on the folder and defined the mail address of this folder to a certain user as a additional address.
the result is, that now this user gets the mails according to the new assigned plus the mails that are wrongly assigned.
example:
mail enabled pubilc folder with address public@
user1@
user2@

mail for user1@ is wrongly directed by exchange??? to public@
next time same external sender is correctly directed to user1@

after disabling mail functionality for the public folder and adding the mail public@ to user2@:
mail for user1@ is wrongly directed by exchange??? to user2@
next time same external sender is correctly directed to user1@

I have definitely no idea cause in the Exchange Message Tracker the wrongly assigned mails already appear with the wrong "to" address.

If I open the mail from the public folder in outlook I definitely have the mail address user1@.

I appreciate any ideas leading to a solution of this weired symptom.

greetings