mail fail to outer mail_address

[xun]

#  username:Linux
#  send a mail to myself
mail -s test Linux
#  check for this mail
mail

when I send a mail to myself ,I can find this mail and it works well.

However,when I send a mail to a outer mail address,such as 163
it doesn't work.
I have trouble with it for days ,I need your help.

 

[VitalkaDrug]

Which outer email do you try to use exactly? What is 163? 163 is not valid email address.

 

[xun]

Which outer email do you try to use exactly? What is 163? 163 is not valid email address.

the above is my email address. when I excute the above and log in my email, i can't find the email.

Howver,when I excute the above,I can find the mail in the mailbox.

And the Mail Transfer Agent(MTA) of my local OS is "sendmail".
Thanks