tcp_deferred_ack_interval - TCP parameter Outlook dealy problem

We have Sun servers running Solaris 8.
We have a problem with MS outlook or outlook express sending email
with a large attachment.
It takes much more time compared to other mailer software like Eudora.
For example, Outlook took almost 1 minutes sending 3MB attachmet.
Eurora only took 6 seconds sending.
We talked Microsoft engineer that they said there is overhead.
Outlook has to get an ACK for each frame it sends out.
We changed tcp_deferred_ack_interval from default value 100
milliseconds to 10 milliseconds. Time dramatically reduced from 1
minute to 9 seconds.
But we are not sure we can change this parameter.
or If we can change, what is recommended value for this parameter? or
If changed 10 milliseconds, is there any potential problem we will
encounter?
Please give us some idea.