all I do is sending out bunch of emails and my server's load gets up to 900s, any ideas where to look?
Load more detail needed before anyone can give you a proper answer.
What do you see in
How many e-mails are 'a bunch'? Are they ones produced in a batch through some automated means? What mail server software are you using on the server?
It would help if you posted you Xen configuration file, however my immediate guess is that the dom-u was given insufficient memory. Is this HVM or paravirt? Please provide some more information.
Loads like that usually mean I/O bound processes ... so some details about your network configuration (bridge / NAT) on dom-0 would also help.