I am using XOOPS 2.0.15 for several days, but yesterday some user reported that they counld not get activation email from the server. However, I, as the webmaster receive emails about new registrations. So I just tried to "mail users" from the administration menu and got these error messages:

Language string failed to load: execute/usr/sbin/sendmail
Could not send mail to ningxu.
Language string failed to load: execute/usr/sbin/sendmail
Could not send mail to Jessie.

P.S. I am using phpmail as the mail sending method.