Migrating my mailhub from 4.9 to 5.x

[Frank BONNET]

Hello

I am in the process to migrate my mailhub ( ~3000 mailboxes ) from 4.9
to the next 5.x release to be able to use native nss_ldap and pam_ldap.

I have tested the 5.2.1 version on another smallest machine and most
of my needings are working.

BUT

I need also to change the webmail I use actually as it seems to be not
supported anymore , this is IMHO working on the Roxen http server.

I need a webmail that works with uw-imap ( mandatory ) and the maildox
format as I have to keep a large amount of mailboxes and don't want
want to use the maildir format for now.

Any recommendations on the webmail I should use ? 
I have tested some on my spare machine but really don't know how they 
will work under a "production load" 

The machine is a HP/Compaq Proliant GL380 with 2 Gb RAM 
and a lot of disk space.

Thanks in advance.

Frank