SMTP Proxy Server is a server that can be used to debug an actual SMTP server. Although many SMTP servers log SMTP communication to log files, these log files are hard to work with if you are looking to track a specific email. SMTP Proxy server is a very useful utility if you are trying to debug problems with your email server. It can tell you:
Who is sending emails to your server
Who is the sender
Who are the recipients
Did your server accept the message
By setting up a SMTP proxy server that allow to see all traffic between sender and receiver, you can find out if the messages are going through the way they should be.