I've figured out that 20% of the confirmation emails are not received by my clients. I use a website hosted at Watsnel.nl. Mails send to ziggo.nl, xs4all.nl and kpn.nl are not received. Which sender is used by Form Builder in this case. I assume the sender is on a blacklist at these providers, what to do next?

Thank you.

The form is on www.qtouch.nl/inschrijven.html. The .fb is now attached.
I do not use S-drive, but I use the exported files. With S-drive there is no issue, but I need the exported files because of extra php and because I use a database.

I have figured out that the email adresses are not on a blacklist. The confirm emails just disappear. Does anyone have some information what could go wrong, is their a log about send messages?

Have you asked your host about it? If it works sometimes and not others It's a host issue.

I have just asked them. And I have to fill in the user.cfg.php.
I just did, but I still don't receive the email on these email addresses, so probably still some minor glitch on my side.

$user_config['mailer']['service'] = 'localhost';
//$user_config['mailer']['service'] = 'smtp';

// Does the remote SMTP server need authentication?
$user_config['mailer']['smtp']['auth'] = true;
//$user_config['mailer']['smtp']['auth'] = false;

// Use this if the server needs authentication: I ENTERED THESE TWO IN MY SCRIPT
$user_config['mailer']['smtp']['user'] = 'emailadres';
$user_config['mailer']['smtp']['password'] = 'password';

// The address of the server: I ENTERED THE FIRST ONE IN MY SCRIPT
$user_config['mailer']['smtp']['host'] = 'mail.server.com';
//$user_config['mailer']['smtp']['host'] = 'smtp.gmail.com';

// Connection parameters for a plain SMTP server
$user_config['mailer']['smtp']['port'] = '25';
$user_config['mailer']['smtp']['secure'] = '';

// Connection parameters for a SMTP server that supports secure connections, such as smtp.gmail.com
//$user_config['mailer']['smtp']['port'] = '465';
//$user_config['mailer']['smtp']['secure'] = 'ssl';

Are the email addresses on your host?

Yes, the email address from this script is on the host. The email address which should receive an email can be anyone somewhere.