I am using Nginx as an Email reverse proxy.
The email client sends a request to the nginx, the nginx fetch the WHOLE email(message) from exchange server, and only then manipulates it and sends it back to client.

Email Client<---->Nginx<----> Office 365

Everything is working great, besides the following problem, the Email client has a timeout of 30 seconds,
but sometimes it can take the Nginx to download the whole email more the 30sec (if it has a big attachment).

My question is as follow:
In the meantime can the nginx send 'something' to the client to keep him aware that something is being downloaded ?
Right now I have an ugly patch that sends on the open socket 'blanks' to the client every 20 seconds, until the nginx can send him the whole file.

Thanks