The thing is that mail() didn't work on Xampp before but since I updated to Xampp 5.6.3 (on mac) it suddenly did.
But not all emails would receive it. My mail on gmx.net wouldn't accept the mails but my mail addresses connected to my webhotel did.
But I use phpmailer https://github.com/PHPMailer/PHPMailer for sending out my mails because when you send out a lot of emails mail() opens and closes the connection on each call but phpmailer can use smtp (for instance you can use gmail, it is slow though) so you can send out many mails in one go.
Say, if you are sending out a 1000 mails mail() is not a good choice.
Edit: Example of using phpmailer. (my webhotel also had a smtp server I could use. I just had to ask them about it and get their settings and port no. On my webhotel there were no login requirement but the emails sent should have an email address connected to the webhotel in the from field and it didn't work locally so gmail was the better option although the authentication makes it slow to send an email this way)
require_once('../class.phpmailer.php');
//include("class.smtp.php"); // optional, gets called from within class.phpmailer.php if not already loaded
$mail = new PHPMailer();
$body = file_get_contents('contents.html');
$body = eregi_replace("[\]",'',$body);
$mail->IsSMTP(); // telling the class to use SMTP
$mail->Host = "mail.yourdomain.com"; // SMTP server
$mail->SMTPDebug = 2; // enables SMTP debug information (for testing)
// 1 = errors and messages
// 2 = messages only
$mail->SMTPAuth = true; // enable SMTP authentication
$mail->SMTPSecure = "tls"; // sets the prefix to the servier
$mail->Host = "smtp.gmail.com"; // sets GMAIL as the SMTP server
$mail->Port = 587; // set the SMTP port for the GMAIL server
$mail->Username = "yourusername@gmail.com"; // GMAIL username
$mail->Password = "yourpassword"; // GMAIL password
$mail->SetFrom('name@yourdomain.com', 'First Last');
$mail->AddReplyTo("name@yourdomain.com","First Last");
$mail->Subject = "PHPMailer Test Subject via smtp (Gmail), basic";
$mail->AltBody = "To view the message, please use an HTML compatible email viewer!"; // optional, comment out and test
$mail->MsgHTML($body);
$address = "whoto@otherdomain.com";
$mail->AddAddress($address, "John Doe");
$mail->AddAttachment("images/phpmailer.gif"); // attachment
$mail->AddAttachment("images/phpmailer_mini.gif"); // attachment
if(!$mail->Send()) {
echo "Mailer Error: " . $mail->ErrorInfo;
} else {
echo "Message sent!";
}