By Alexei Oreskovic SAN FRANCISCO (Reuters) – Yahoo Incs email service has been unavailable to some users since Monday night due to a hardware problem that the Internet company said has been harder to fix than it expected. Yahoo believes the problem will be fixed by 3 p.m. PST Wednesday, according to a post on Yahoos official Tumblr blog by the companys senior vice president of communications, Jeffrey Bonforte.