[Exim] Error 554 from hotmail.com

Hi all,

i use Exim 3.35 (Debian Linux) ad i've trouble
with mail sent to hotmail.com.
When i send a message to any hotmail address
I get a 554 Transaction failed error from remote
host.

Reading all around internet forums i think
that error is in the "helo" command that
my server send to hotmail.com.

How can i "rewrite" the helo string?

Thanks,
Giordano