I tried to use WP Mail SMTP. There’s the error message:
WP Mail SMTP has detected incorrect "wp_mail" function location. Usually, this means that emails will not be sent successfully!
It looks like the "Newsletter, SMTP, Email marketing and Subscribe forms by Sendinblue" plugin is overwriting the "wp_mail" function. Please reach out to the plugin developer on how to disable or remove the "wp_mail" function overwrite to prevent conflicts with WP Mail SMTP.
Current function path: /var/www/vhosts/hosting175068.ae88e.netcup.net/moinhaus.de/httpdocs/wp-content/plugins/mailin/sendinblue.php:324
The settings in my sendinblue account and plugin are correct.