There is a bug in the CDOSYS section of inc_mail.asp if cdoSendUsingPort is not defined anywhere on your server.
on line #128 find the following:
Flds("http://schemas.microsoft.com/cdo/configuration/sendusing") = cdoSendUsingPort 'cdoSendUsing enum value = 2
and change it to this:
Flds("http://schemas.microsoft.com/cdo/configuration/sendusing") = 2 'cdoSendUsingPort