Where can I see an example of writing a POP3 mail server?

Question

Java

On the Internet, everywhere I find only how the client writes. I would be happy if someone shares a link)

Answer 1

If I understand correctly that you are interested in implementing a POP3 server in Java, then for example, you can look at the
source code of Apache James , the
source code of Jes
jemailserver
and further down the list ...
if something is not clear, ask specific questions.

Answer 2

well, go to the raws of any open source pop3 server