PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
prefix email: <http://davide.eynard.it/rdf/email#>

select * 
where{
	{ ?x email:server "server1" ;
         email:subject ?subj } 
	UNION 
	{ ?x email:server "server2" ;
         email:subject ?subj }
	UNION 
	{ ?x email:server "server3" ;
         email:subject ?subj }
}