Problem in Sending Email through an application using server relay

we have an application where we are trying to send the email thorough a SMTP server that allows email relay to some specific server machine( machine whose IP address is added for relaying). We are able to send the email through that server using either command prompt or the exact application inside it.

The problem which we face is we just connect to the server and use it on the normal machine and that is somehow not working.

The assumption which we have right now is that the application no matter what connected to the server will try to send the email through the local machine and that is not allowed. So the error we get from outside is “Mailbox Unavailable” .

The way I feel it could be done is the application working as if it was inside the server or the feature of the email working through the server or if there could be some more alternatives to do it.

PBA the problem screenshot and please revert if you can help and I will appreciate that