I am presenting this problem also to out internet host. I installed TikiWiki via Fantastico and I am unable to recieve any emails via the Contact Us page. "admin" is the email contact user in the General setup and the email is setup correctly for admin in the user preferences.
The contact form submits and says the email was sent, but it never gets to the recipient. I put a plain PHP page on the site with a simple email script in it and the email went out fine, so I'm pretty sure it isn't some setting on the server causing the problem.
Does TikiWiki do something special with outgoing emails? I couldn't find anything close to this problem in the forums.
My host suggests that TikiWiki is sending the emails as "nobody" Their CGI email process won't let me send as anyone other than myself so I figure this may be the issue. Now I have to find out if that can be changed in TW