Question Changing main mail address

[Milow]

It is possible to change the main mail address of an user (the field is writeable) but it leaves you with an error. The address will change but not the name of the users folder, so the postbox is broken.
The question is, is the error the bug or the possibility to change the address?
Why would I do this? The main mail address is the one used as the sender of an auto reply but it is not the one the user uses as his main address in communication. So he want one of the alias as the main address.
Any suggestions to bypass the problem?
Thanks in advance.