[Exim] Problem sending to MS Exchange

Hi.

Does anyone know why sometimes I have problems sending from either sendmail
or exim to MS exchange. I heard mentions of creating an SMTP address
for the Exchange user but those sometimes work. I have such an arrangement
and exim is able to send to it but not the sendmail process. I realize
this is not an exim problem but since there're email experts here, I wonder
if someone could spare a few cycles and explain the technicals involved.

Thanks.

C