For a long time I used to use smtp-sink — http://www.postfix.org/smtp-sink.1.html
$ smtp-sink -u nobody -R /tmp/smtp-sink -d "maildir/%Y-%m-%d/mail.%H.%M." 127.0.0.1:25 1024
Until I joined a team that didn’t find cli as fun and preferred GUI tools. And that’s when I found mailcatcher. It’s solid and just works.