Before the questions, a quick word on the philosophy of the mailserver.
The Mailserver Virtual Appliance is designed to be the best dedicate mailserver out there based on Open Source Software. It is designed to be right for most users, not all users. There will always be advanced or specialised configuration which is difficult to implement, or not applicable to most installations and instead of making the user interface more difficult than it need to be - it's supported to tweak the mailserver in any way that you want by following the instructions on the AdvancedConfiguration page.
It also needs mentioning that tweaking the mailserver beyond it's original functions is only available for the activated / paid version.
Most of this will require console access and is therefore only supported on the activated / paid version.
On the Mailserver, the mails are stored in Dovecot's mailbox format in the directory /var/mailserver/mail/_domain_/_user_ (i.e. john.doe@example.com will be stored in /var/mailserver/mail/example.com/john.doe). Depending on where you migrate from, you can follow the instructions at the Dovecot Migration page for most migrations.
If your specific mailserver is not supported, the easiest way to migrate is probably going to be to setup the old mailserver and the new Mailserver in parallel and get the users to configure both mailservers and then tell them to simply drag and drop all mails they want to keep on to the new mailserver. This will only work if the users configure the new Mailserver server to use imap.
The RoundCube webmail interface chosen is installed as is. There's no further tweaking being performed or planned, so whatever is supported in the latest released version of RoundCube is what supported in the Mailserver as well. Currently, this does not support LDAP and please see this ticket for votes on LDAP integration into RoundCube.
The easiest way to filter messages is by using an automatic name extension described in the documentation here. If that doesn't work, it is also possible to use Sieve scripting. There's currently no web interface for the server side filtering.
Dovecot is compiled to use sieve so if you place a script in `/var/mailserver/mail/_domain_/_user_/home/.dovecot.sieve` it will be used to filter emails. Please see http://wiki.dovecot.org/LDA/Sieve for some examples on how to use Sieve in Dovecot. Please be aware though that setting a Vacation message in the User account settings page will overwrite the .dovecot.sieve file.
No. The Mailserver is not about freedom of choice for different components. Neither will there be a version with Qmail instead of Postfix or Courier imap instead of Dovecot. There will only be one version of the Mailserver and it will be the best that it can possibly be, with best of breed components every step of the way.
This can be safely ignored.
There's actually two different VMware tools installed in the Mailserver. The OpenBSD team has started building their own kernel extension (vmt) that currently only helps feeding time from the host to the guest. This is included in the VMware version of the Mailserver starting from version 4.4. The other is the FreeBSD version of the VMware tools that perform two functions - they shut the guest down and reboot gracefully when “shutdown” and “reboot” are selected from the VMware host.
Hopefully in future releases of OpenBSD the vmware support is extended to reboot and shutdown the guest from the kernel driver. When that happens the FreeBSD VMware tools will be removed from the distribution.
If you find that the time in the Mailserver is creeping, you're probably having the problem outlined here: http://kb.vmware.com/kb/1591. In version 4.4 and later there's a vmt kernel driver being distributed that should also assist with this problem.
This functionality is not built in to the Mailserver as a standard feature, but can be achieved using fetchmail. You will need a licensed Mailserver to install fetchmail, and then run on the console:
export PKG_PATH=http://mirror.planetunix.net/pub/OpenBSD/`uname -r`/packages/i386/ pkg_add -i fetchmail
Please see the Fetchmail man page for instructions on how to configure Fetchmail.