[Dovecot] migrate from cyrus with virtual domains

[Andrey Kolbasenko]

> Check out the auth_username_translation and auth_username_format variables
> in dovecot-example.conf.  One of those might allow you to do what you want.
>
> Bill

Thank's for reply, but I think this variable <auth_username_translation> 
switch all dot to @, not only first. And I can't find <auth_username_format> 
variable in dovecot-example.conf and all docs about dovecot.

Andrey