Either your system administrator has disabled the mail() function in /etc/php.ini or
The system administrator has disabled the mail() function in your /~$HOME/php.ini, or
/bin/sendmail doesn't exist, or the apache user does not have privileges to read/execute.
If you have shell access, see if you can telnet to port 25 on mail.yahoo.de
echo "this is a test" | mail email@example.com
If you receive that test, your username does have access to use /bin/sendmail.