If you see messages in the in Exchange 2010 Queue Viewer unreachable Domain queue and email are not going into Mail Enable Public Folder there is a problem with your mail enable public folder.
This is the error in the Queue Viewer
Select the Public Folder that is not receiving email and click on Mail Disable (Note
If the Public Folder is using alias email address take note of it before doing this)
After you click On the Mail Disable, Right Click again and Mail Enable the Public Folder.
Right Click again and Click on Update Content.
That should fix the problem.