Question on connecting a SMTP server to Exchange 2013

I have a 3ed party software that has a built in SMTP server that coworkers want to use.

However, there is no where to input credentials to access a mailbox within that server.

Therefore, User A can use it to send emails from User B's mailbox.

Are there any settings in exchange to prevent this?

This is all I have to work with in the 3ed party software.

Thanks!