[Dovecot] unknown namespace with Mac OS client

[Francisco Reyes]

Ken Dail writes:

> That's pretty much the situation I saw several different clients  
> behaved poorly with this kind of setup. Mail.app is just broken Maybe  
> Timo can put in a workaround for ignore invalid namespaces provided  
> by broken clients.


So few of our clients use mail.app that it just didn't make sense to 
inconvenience the majority trying to help just a handfull of clients.

Given that we have many server it is not too much of an issue to make sure 
we leave a couple of courier boxes behind as we migrate.. but for people 
who only have one server, this is likely a problem.