Nginx Module Reference manual: Message agent module (mail proxy)

Nginx can proxy IMAP, POP3, and SMTP protocols.

Instructions

Proxy

Syntax: Proxy on | Off
Default value: Off
Working with fields: Mail, server
Sets whether mail agents are enabled.

Proxy_buffer

Syntax: Proxy_buffer size
Default value: 4k/8k
Working with fields: Mail, server
Sets the buffer size for the proxy connection, which defaults to paging size, which may be 4k or 8k depending on the operating system.

Proxy_pass_error_message

Syntax: Proxy_pass_error_message on | Off
Default value: Off
Working with fields: Mail, server
You can pass the error authentication http://www.aliyun.com/zixun/aggregation/38653.html "> information from the backend to the client, usually if the Nginx authentication is passed, then the backend error message cannot be delivered to the client.
However, some POP3 errors in the correct password answer, such as Communigatepro notify the user that a mailbox exceeds the capacity limit (or other events) will issue a periodic error in the authentication, in which case it is necessary to open the proxy_error_message.

Proxy_timeout

Syntax: Proxy_timeout time
Default value: 24h
Working with fields: Mail, server
Sets the timeout time for the proxy connection.

Xclient

Syntax: Xclient on | Off
Default value: On
Working with fields: Mail, server
Whether to enable the Xclient command for SMTP back-end connections, this allows the backend to enforce client connections on Ip/helo/login.
If Xclient is enabled, then Nginx is first forwarded to the backend:

And then: