Hi,
relay control is set in smtpd.cfg. See this recent thread on the subject:
http://www.scalix.com/community/viewtop ... =smtpd+cfg
You can test that with telnet or with a mail client.
1600 users are no problem at all, even on a single box.. Depending on the usage pattern (are they 100% Outlook users, 100% IMAP , n % POP, n% SWA?) one needs to adjust the hardware parameters accordingly. Let us know what mix there is and we can provide a recommendation. Generally speaking I would go for a dual Xeon/Opteron with 8-12GB of RAM with plenty of diskspace.
UTF-8 support is planned for release 11. We do provide some character set other than ISO 8859-1 as of now, but let us know the exact requirement and we can give you more information.
Cheers,
Sascha.