connect mailcow with wordpress

Hi,

my mailcow server is working as fine under the subdomain mail.fqdn (mail.example.com)
and my Wordpress Shop is working under the fqdn (example.com) So I installed the plugin FluentSMTP in Wordpress.
I choosed othersmtp an my settings are:
Sender Settings:
From Email: info@example.com
From Name: My Name
I checked the Boxes - Force From Mail - Set the return-path to match the From Mail

SMTP Host:
mail.example.com
SMTP Port:
465
Checked the Box: SSL
Checked the Boxes: Use Auto TLS and Authentication (username and password from Mail)
Store Access Keys in Database
SMTP Username: info@example.com
SMTP Password: the password of this mail account.

I got this Error in the Logs:
{
“code”: 423,
“message”: “SMTP Error: Could not connect to SMTP host. Failed to connect to server”,
“errors”: [
“SMTP Error: Could not connect to SMTP host. Failed to connect to server”
]

The mailcow is on the same server with a relay proxy in apache so its reachable under mail.example.com.
That did I wrong? Thank You very much for help!

Greetings