Email Reverse Proxy issue

[omercz]

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

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,250534,250534#msg-250534