How to know whether I am able to send email from unix server or not?

How to know whether I am able to send email from unix server or not?

I ma trying to send mail to my gmail id using command

sendmail -s "Subject" abc@gmail.com
Hi
How are you?
.

It showed that You are not allowed to send mail.
My question is there any file which contains the list of allowed oe not allowed users?

Regards
Siba