nginx mail starttls ip based

[Marco]

Hello,

 I'm new in nginx world. I would like to configure a single nginx server
to proxy imap/pop servers with starttls option.

I would like that the "starttls" could be "on" on a list of known safe IPs
and "only" on other IPs. Using the same listen port and protocol.

Could you tell me how to achieve this?
Maybe, is there a way to set the starttls in auth phase?

For instance, let suppose  I have configured this:

  server {
    listen     110;
    protocol   pop3;
    starttls   on;
    proxy      on;
  }

  server {
    listen     110;
    protocol   pop3;
    starttls    only;
    proxy       on;
  }

I would select first or second server block using Client-IP header.
Is it possible?

Thanks a lot.
Best Regards
Marco