Sendmail vs Mailx

sendmail has the 'intelligence' to figure out how to send the mail to a recipient not on the local machine - provided the machine has access/can route to the network where recipient is located. In order for mailx to do this, you need to use /etc/aliases ( /etc/mail/aliases ) 

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 Reference
 https://forums.oracle.com/forums/thread.jspa?messageID=8258664