Wow, this get's very passionate...

So let's try to add a bit more information here.
1) I've talked this through with our engineering team a couple of times - they claim that Outlook does not offer any API interface that allows us to properly implement this. Always remember - Outlook exposes certain APIs that we use to connect into it. The integrated POP/IMAP client only works with a PST-based storage model; Scalix is built around a thought of a "primary default Store" MAPI provider, which is server-based - we don't keep data in PST files and on the client for many reasons. Therefore, the two things are mutually incompatible and we haven't found a way to change this behaviour, possibly none exists. The fact that this is possible with Exchange is no proof here as Microsoft might have done something within their core code that is just not possible for external plugins. They've done this in a couple of areas.
Therefore it seems that it's not a question of what we do support, but what we can support.
2) The requirement might look common when reading through the posts, but in some environments it is pretty much the opposite. Bypassing all server-side email security checks (Anti-Spam, Anti-Virus, invalid content) by directly downloading information from potentially external servers into a desktop client bypasses the email server which usually includes some policy enforcement (the above plus archiving, server-based storage, backup/restore); therefore, most larger organizations would probably block POP or IMAP traffic from the clients at their firewall. If at all such accounts must be included for "official" purposes, a server-side solution such as fetchmail is strongly encouraged.
nevertheless, we're tracking this, but honestly I don't see a solution on the horizon anytime soon, for the reasons stated above.
HTH,
Florian.