Php mail function not working after plesk update

Hi,

After updating Plesk from 11.x to 12.5, the mail() function doesn't work anymore.
I can sent mail with Outlook (imap) and webmail, but on a website I can't.
I tried it with the mail() function, but also the phpmailer class don't work.
The mail() function always returns false. (no errors)

After searching hours for a solution, I hope you can help me?

Greetings