Prohibit dots in folder names

[Marek Grondecki]

Hello Dovecot Community,

I am using Dovecot 2.2.13-12~deb8u1 (Debian Jessie).

separator = /
listescape plugin is NOT active
LAYOUT=fs is NOT active

I would like to prohibit a creation of folders with dot "." in the names 
-
so creation of a folder "foo.bar" should NOT be possible.
Currently, when I create "foo.bar" in Outlook 2013 it will be created 
but won't be
synchronised with the mail server - Outlook shows it as "foo.bar (Only 
this computer)".
Ideally the user should receive an error message informing that "the 
folder foo.bar
could not be created."

Thank you for your help.

Regards,
Marek Grondecki