one mail user id multiple instances
 

i want to configure mail server with two email id
a@example.com ,b@example.com
here a will only send mail to b
b will only receive mail but will never reply.

here it should be only one way traffic from a to b.

a@example.com user id will be used to send data to b@example.com

there will be multiple instances of a@example.com and all will send mail at a time.

Ok, easy enough. Which mail server are you using?

postfix
 

i am using postfix

Setup your postfix system according to your needs:

http://www.postfix.org/SOHO_README.html

The create a restriction class allowing a@example.com to send, but not b@example.com

http://www.postfix.org/RESTRICTION_CLASS_README.html