Thank you Lukasz,
but I don’t understand the examples completely.
What I’am doing is:
function contactform7_before_send_mail( $WPCF7_Submission ) {
// here I get the fields-contents and store them into a db
// if the email-adress is not in the db, the OK-Message of cf7 is send.
// if the email-adress is already there, I want a custom message with my own text.
}