SMTP Port 25 Issue on Nginx Mail Proxy

Question

I set up an Nginx mail proxy server and configured it to distribute to several mail servers as a front-end server. However, I need to open port 25 in the configuration; otherwise, I cannot receive mail. When I open port 25, I have to enable the smtp_auth plain login feature; otherwise, if someone knows my email address, they can send mail without a password.

When I enable the smtp_auth feature, I can't receive emails when someone sends an email to my address from outside.

For example, when I try to send an email from Gmail to my server, the message returned by Gmail mail delivery is as follows:

The remote server's response was: 530 5.7.1 Authentication required

How can I resolve this issue?

/etc/nginx/nginx.conf (mail {} partition)

mail {

   server_name mail.proxyserver.com;
   auth_http localhost/auth/auth.php;
   pop3_capabilities "TOP" "USER" "UIDL" "PIPELINING" "SASL";
   imap_capabilities "IMAP4rev1" "UIDPLUS" "IDLE" "LITERAL+" "QUOTA";
   smtp_capabilities "SIZE 53477376" "8BITMIME" "ENHANCEDSTATUSCODES" "PIPELINING" "DSN";

   proxy on;
   proxy_pass_error_message on;
   proxy_timeout 300s;

    starttls                   on;
    ssl_protocols              TLSv1.2 TLSv1.3;
    ssl_ciphers                HIGH:!ADH:!MD5:@STRENGTH;
    ssl_session_cache          shared:TLSSL:16m;
    ssl_session_timeout        10m;
    ssl_certificate            /etc/letsencrypt/live/mail.proxyserver.com/fullchain.pem;
    ssl_certificate_key        /etc/letsencrypt/live/mail.proxyserver.com/privkey.pem;
    ssl_dhparam        /etc/ssl/certs/dhparam.pem;

    server {
            listen     25;
            listen     [::]:25;
            protocol   smtp;
            starttls   only;
            smtp_auth  none; # my problem :(
            xclient    off;
    }

    server {
            listen     465 ssl;
            listen     [::]:465 ssl;
            protocol   smtp;
    }

    server {
            listen     587;
            listen     [::]:587;
            protocol   smtp;
            starttls   only;
    }

    server {
            listen     110;
            listen     [::]:110;
            protocol   pop3;
            starttls   only;
    }

    server {
            listen     995 ssl;
            listen     [::]:995 ssl;
            protocol   pop3;
    }

    server {
            listen     143;
            listen     [::]:143;
            protocol   imap;
            starttls   only;
    }

    server {
            listen     993 ssl;
            listen     [::]:993 ssl;
            protocol   imap;
    }
}