Email SMTP Port 25 SSL Error

Your software
My Mautic version is: 5.2.2
My PHP version is: 8.3.15
My Database type and version is: Mariadb 10.5.27

Your problem
I have just installed Mautic 5.2.2 for the first time and can login in ok.
I have setup email for SMTP port 25 and when I press test I get an SSL Operations failed error.
tls_process_server_certificate:certificate verify failed.

SMTP port 25 should be sending emails without encryption so I’m surprised to get a a tls error. There is no field to select encryption type is none. Therefore mautic must have encryption coded based on the port.

I’m using an on-premise email server with a self signed certificate. I can telnet to this server with the logon credentials and it works fine. Other servers are using it in a similar way successfully

Is there a way to modify the mautic code to fix this problem as I wish to connect to this SMTP port 25 server.

Thanks.