i cant send mail

hi i have a hp-ux B.11.11 U which i connect from telnet.

i want to use mailx command to send mail. I configured /etc/hosts and add the ip of an mail server computer. Sendmail is installed and when i try to send mail, it doesnt show any error,
but it returns to my mailbox as returned mail. This (returned) mail says the problem is unknown host.
How can i solve this problem.
if you want more detail i try to explain...

thnx

this is my command

mailx -s "try" halil@mymail.com

Where do you see this "unknown host" ? Can you post the whole error message if viable ? Is possible that you misspelled the hostname ?