SMTP server Unable to relay for email error

[static]

Hi everyone,

I am facing the error that the system is unable to send relay email.
Before I post my code, please correct me if my understanding on the SMTP is wrong on below scenario :

A - the server where the exchange server was set up
B - the server where I configure Relay (I have given the Relay port 127.0.0.1 only)
C - my computer where I run the program to send email.

For the relay configuration, i created new receive connector and give B's IP Address.

In my program, which IP address do I need to use for sending email (A or B )?
The exchange server was set up in A , so if i give the A ip address, does it relay to B ?

Thanks in advance,
static