Simple SMTP Server using Apache James (Netty) and Spring Boot which just logs sender, recipient and size for every message.
It is very easy to use an existing Apache James Hook or implement your own Hook. Also see https://james.apache.org/server/dev-provided-smtp-hooks.html
mvn spring-boot:runtelnet localhost 10025
Trying ::1...
Connected to localhost.
Escape character is '^]'.
220 LT02064.local SMTP Server (JAMES SMTP Protocols Server) ready
ehlo a
250-LT02064.local Hello a [0:0:0:0:0:0:0:1])
250-PIPELINING
250-ENHANCEDSTATUSCODES
250 8BITMIME
mail from:<hello@world.com>
250 2.1.0 Sender <hello@world.com> OK
rcpt to:<hello@world.com>
250 2.1.5 Recipient <hello@world.com> OK
data
354 Ok Send data ending with <CRLF>.<CRLF>
Subject: hello
world
.
250 Command accepted
quit
221 2.0.0 LT02064.local Service closing transmission channel
Connection closed by foreign host.