With AIX 4.x, by default you are not able to send mail to hosts you cannot ping on the Internet. To enable sendmail, to forward mail to hosts that cannot be pinged, you will need to uncomment the "OK MX" statement in your "/etc/sendmail.cf" file. In other words, remove the # symbol before the "OK MX" statement. Below is an example showing what that section of the sendmail.cf file should look like.
# You may use any combination of these, although it is recommended that
# you specify MB if MR is specified. For example, "OK MG MX" would enable
# the use of Mail Group and Mail Exchanger resource records.
#
# The default is not to use a name server for resolving recipients.
#
OK MX
# Name of the codeset for Japanese network.
# Sendmail will convert mail from/to the codeset of a locale to/from
# the codeset of the network specified with this option.
# Note: The OOcodeset option must also be set when using the Obcodeset
# option.
# Obcodeset
This is the only modification you need to make. For this change to take effect, you need to rebuild the sendmail configuration database and refresh the sendmail subsystem. The commands below are an example of how to do this:
/usr/sbin/sendmail -bz
refresh -s sendmail