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Oh, if you do have a dynamic IP for your server then even with SPF you will have a different problem. MSN/Hotmail (and AOL) block email from servers without reverse DNS and, basically, you can't implement reverse DNS for a dynamically allocated IP. More about this at the excellent
DNSstuff.com site.
What you need to do is set up XOOPS to send mail using your ISPs SMTP login (as I presume you have already). You can then create a SPF record
it-hq.org. IN TXT "v=spf1 include:myisp.com"which will make any server which is valid for sending mail from myisp.com valid for sending mail from it-hq.org.
Beware that you may be infringing your agreement with your ISP by using its SMTP as a mail relay like this, although in practice you are a lot less likely to cause them a problem than a home user with a worm. However, the 'right' way to do it is either to get a line with a static IP with reverse DNS provided by the ISP, or to use a hosted server.
Good luck!