I am new to Unix and i want to know when using mailx can we change the sender's name from the default one.
No, you cannot. You must use the sendmail instead, and include the -f option to set the sender.
when i m using the command sendmail it gives the error sh: sendmail: not found
Test-PC:~ # ll /usr/sbin/sendmail
-rwxr-xr-x 1 root root 18864 Sep 9 2005 /usr/sbin/sendmail
see what that returns