SMTP simple implementation guide

Hello everyone, I'm trying to implement SMTP protocol in java, a simple implementation. I've looked up some repos on github but all I've found were very large projects.

I wonder if you can guide me on what should I do? like a book suggestion or an online course? I want to be able to demonstrate my understanding of the protocol by writing a simple client and server that run on a local machine and send emails to each other.

thanks.