How to increase ACK speed for TCP packets in Platform SDK Windows Sockets.

I have implemented a Receiver-SMTP server for file transferring between a PC
and a voice-mail machine which already had a Sender-SMTP inside.
It works beautiful BUT slow.
The voice-mail machine sends TCP packets of exactly 1400 bytes and waits for
ACK; (due to hardware restrictions it is not able to do it with a higher
window level, I mean, it is not able to keep on sending packets until the
arrival of that ACK).
But the PC seems to be waiting for longer packets, so it waits for 200
miliseconds (time-out) before giving that ACK.
I'm using Platform SDK Windows Sockets.

My question is:

Is there any way to fit the TCP packet size I'm waiting, so that after
receiving that exact amount of bytes the ACK is immediately fired? Or is it
configured by the operating system and not programmatically changeable?
Is there any other way to increase speed for that ACK sending?

Thank you!!

Ricardo Vázquez.
Madrid, Spain.