Logging when proxying SMTP / IMAP / POP sessions

[Luis E. Muñoz]

Hi there,

For an email proxying setup using the open source version of Nginx, I 
have a configuration stanza along these lines:

```
⋮
mail {
   server_name              mail.DOMAIN;
   auth_http                127.0.0.1:8080/auth;
   proxy_pass_error_message on;

   imap_capabilities        "IMAP4rev1" "UIDPLUS";
   imap_auth                login plain;

   server  {
     protocol imap;
     proxy on;

     ssl on;

     ssl_certificate     ⋯;
     ssl_certificate_key ⋯;

     listen [::]:993 ssl;
     listen 993 ssl;
   }
   ⋮
}
⋮
```

Proxying and authentication works as advertised, no issues. However, 
I've been unable to locate any piece of documentation indicating how to 
produce a suitable access log message (ie, a log tying the mail client 
with the backend server its connection was sent to). I've tried the 
`access_log` / `error_log` directives at various places in the 
configuration, to no avail.

HTTP/HTTPS logging works as usual.

Given how I've not seen any mention to logging in the various documents 
that describe SMTP/IMAP/POP proxying, I believe the functionality for 
this does not currently exist. This logging is very important for 
troubleshooting and support.

Is there something I'm overlooking? If indeed the functionality doesn't 
exist, would it be possible for me to write and contribute the code 
required to add said logging?

Thanks in advance.

-lem
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