Hi Alex,
"localhost" didn't work. Maybe because I'm running on a rented webspace. I tried the smtp server of the company and that works fine now.

I just wonder why sendmail doesn't work as I'm running a links flat file system on the same webspace (different domain) and that works fine with sendmail. Also all other cgis (shopping cart, formmail etc.) work fine with sendmail.

Thank you for your tip.