This was due to a bug (which has since been fixed).  There was never 
an intention to not allow periods in a subfolder name.

Update:



It lets you type in dots when creating a folder, however it doesn't 
let you type in a dot when creating a subfolder. (which might be the 
behaviour on the installation you are looking at)

I see.



Well, some versions of the older interface allow dots but DIMP v 1.29 
2007/11/12 16:53:32 (H3 1.0-cvs)doesn't allow dots.



(Installed with Horde v 1.205 2007/11/12 10:01:25 and IMP v 1.111 
2007/11/08 04:50:59)

No.  Dots create subfolders *if* your delimiter is '.'.  But there is 
no requirement that the delimiter is '.'.  it could be '/' (as in 
UW-imap), or ',', or 'j'.
We had a long discussion about this, but unfortunately the discussion 
took place in a non-public forum.  One of the other points somebody 
brought up (I think it was chuck) is that it is impossible to create a 
container folder in certain IMAP servers unless you allow users to 
type in the delimiter characters.
The previous interface *did* allow you to enter dots (i'm trying it 
out right now).  The only way to prevent you from entering dots would 
be to have some sort of javascript event observing code, and we have 
never had this in dimp.

Dots create subfolders. Jan argues that this is a "feature" but I'm 
thinking about the majority of users who would type in a dot and be 
startled to see (or not notice) a subfolder.



Also multiple dots can cause the IMAP server to reject the name which 
can frustrate ordinary users.



I propose the interface not allow users to type in dots like the 
previous interface did. (advanced users can still paste in dots). 
There is already a straight-forward familiar way to create subfolders.

What's wrong with typing in a dot?  It's a perfectly valid character 
for use in a folder name.  See RFC 3501[9] (definition of "mailbox").

The new fade-in black screen that covers the entire page when the user

attempts to rename or create a folder allows the user to type in dots

(.).



The previous interface didn't let users to do so.



This bug also exists in FRAMEWORK_3 snapshots of horde and IMP