Jason Hendriks on "[Plugin: Postman SMTP Mailer/Email Log] Weird "wp_mail has been declared by another plugin or theme" Error"

Hi. Your diagnostics are suspicious

WordPress wp_mail Owner: /var/www/vhosts/mysite.de/httpdocs/wp-config.php

Open up this file and see what's in it. See if you see something like the following

function wp_mail

That's not part of the normal WordPress distribution.