This module is not built by default, it should be enabled with the --with-mail configuration parameter.

worker_processes 1;

error_log /var/log/nginx/error.log info;

mail {
    server_name       mail.example.com;
    auth_http         localhost:9000/cgi-bin/nginxauth.cgi;

    imap_capabilities IMAP4rev1 UIDPLUS IDLE LITERAL+ QUOTA;

    pop3_auth         plain apop cram-md5;
    pop3_capabilities LAST TOP USER PIPELINING UIDL;

    smtp_auth         login plain cram-md5;
    smtp_capabilities "SIZE 10485760" ENHANCEDSTATUSCODES 8BITMIME DSN;
    xclient           off;

    server {
        listen   25;
        protocol smtp;
    }
    server {
        listen   110;
        protocol pop3;
        proxy_pass_error_message on;
    }
    server {
        listen   143;
        protocol imap;
    }
    server {
        listen   587;
        protocol smtp;
    }
}

Sets an address and a port for for a socket, on which the server will accept requests. Only port may be specified. An address may also be a hostname, for example:

listen 127.0.0.1:110;
listen *:110;
listen 110;     # то же, что и *:110
listen localhost:110;

IPv6 addresses (0.7.58) are specified in square brackets:

listen [::1]:110;
listen [::]:110;

UNIX-domain sockets (1.3.5) are specified with the “unix:” prefix:

listen unix:/var/run/nginx.sock;

The optional bind parameter instructs to make a separate bind() call for a given address:port pair. The fact is that nginx will bind() only to *:port if there are several listen directives with the same port but different addresses, and one of the listen directives listens on all addresses for the given port (*:port). It should be noted that the getsockname() system call will be made in this case to determine an address that accepted a connection.

Different virtual servers must listen on different address:port pairs.

Sets the protocol of a proxied server. Supported protocols are IMAP, POP3 and SMTP.

If the directive is not set, the protocol can be detected automatically basing on the well-known port specified in the listen directive:

• imap: 143, 993
• pop3: 110, 995
• smtp: 25, 587, 465

Unnecessary protocols can be disabled using the configuration parameters --without-mail_imap_module, --without-mail_pop3_module and --without-mail_smtp_module.

Sets a configuration for the virtual server.

server_name hostname;

Sets a name of the virtual server, used:

• in the initial POP3/SMTP server greeting;
• in the salt during the SASL CRAM-MD5 authentication;
• in the EHLO command on the SMTP backend connection, if the xclient command is turned on.

so_keepalive off;

Controls if the “TCP keepalive” mode should be enabled on the client’s connection (SO_KEEPALIVE socket parameter) on the proxied server connection.

timeout 60s;

Sets the timeout which is used before proxying to the backend started.