Send Mail out of a PHP-Apache Container

When trying to send an email with PHP

function sendmailoversmtp($name, $mail, $message, $topic)
It does not work. The following error is logged:

sh: 1: -t: not found
-t -i is configured as sendmail_path in my php.ini. Why is it not found?

My Dockerfile is based on FROM php:7.0-apache

I’ve searched on Google for hours, but couldn’t find a working solution :frowning: