Listens for SMTP connections, accepts them, and writes any emails that come through them into an inbox on the filesystem.
There are 3 uses that I know of for this:
sh
cargo install smtp-dump
sh
date >> logs; bash -c 'setsid smtp-dump </dev/null &>>logs & jobs -p %1'
sh
lsof logs
lsof -i tcp:25
sh
killall smtp-dump
sh
killall smtp-dump; date >> logs; bash -c 'setsid smtp-dump </dev/null &>>logs & jobs -p %1'