==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.6.8
- Deployed with iRedMail Easy or the downloadable installer? Downloadable Installer
- Linux/BSD distribution name and version: Ubuntu 20.04 LTS
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx): NginX
- Manage mail accounts with iRedAdmin-Pro? No
====

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;
    }
}