zimbra as internal without fqdn

[tanoatzim]

Hello,
this post is similar to the many discussed few days ago but that does
not answer to my question.
In my previous post I would like to know if exists a solution to use
zimbra for an internal lan, and setup it with a dumb domain so that
the users could use it to exchange messages between them, and
eventually make an individual setup for users that want to download
mail from their real pop-3 account, post, and work in this way.
Now I would like to know if is possible to try a different solution,
as asked in another thread in these hours (but the conversation
moved to try the fqdn installation): i.e. do not use a domain
at all, and identify internal users only by their username.
Is this possible?

[uxbod]

Why not just use a fake domain and split DNS ? The users will be able to find each other using their username as they will be identified in the GAL. I do not see any problem with this configuration as they can then setup external account collection for either POP3/IMAP services.

[tanoatzim]

Yes, only I would like to know if is possible without using a domain at all.
A thread few days ago do the same question, but no answer reply to
this point. I know split dns could be a solution, but it is important for me
to know all the options I have :-)