Even if I'm inside the box itself & send emails to myself, it was not received too (despite that sending to external emails including gmail was Ok) :

mailx -s "tst to localhost" myunixid@localhost < /etc/hosts

mailx -s "tst to myportaltech" myunixid@myportaltech.com < /etc/hosts

mailx -s "tst to hostname" myunixid@hostname < /etc/hosts

mailx -s "tst to local private IP" myunixid@private_IP_addr < /etc/hosts

su - myunixid

$ mailq /var/spool/mqueue is empty Total requests: 0