I think the version for 5800 XM is not yet stable/mature enough. I've tried to download it by following the procedure described. OK, I could setup my gmail account and receive the SMS.
By the way, If I think the tens times I setup email accounts by just providing user/password/servername, this sounds like a pervert absurd setup. Why cannot a "messaging" application simply be an e-mail client?
Anyway, after all the above, I tried to open the "@ mail" icon, it starts querying the server endlessly and nothing else happens. Worse, even if I prior connect the phone via WLAN, it keeps using the Internet connection from my mobile operator, is this a bunch with operators to suck us money?
Finally, I was trying to see how the messaging application works, simply because the existing mail client in the 5800 has this idiotic thing, that you cannot actually delete a message from the phone only.
You must choose either to delete the local copy, but keep the header in the phone, or to delete the message completely, which means from the server too.
Now, I cannot figure out how the Nokia software developers can possibly think for a second that this can be work well. Maybe they only write "messaging applications", but for their correspondence they still use letter paper and fountain pen...
In any case I have 3 mail accounts, with an average of 4000 (four-thousands...) messages in each. I use the server as a centralised storage for them, and I delete them every 1-2 years, after a DVD backup. Clearly I cannot keep them on a 8Gb phone.
In conclusion, the 5800 seems to have a fairly good hardware, but it also seems that Nokia is mostly interested in equipping it with teenager-oriented applications, such as the media player. Calendar is poor (for instance, you cannot manage multiple calendars), mail quite sucks, with the problem described and without support for HTML/PDF/DOC/XLS etc. attachements.
That's quite frustrating...