Duplicate SMTP traffic for SMTP dlp testing

Take a look at

SMTP Proxy

for some ideas on handling SMTP.

The first thing is that SMTP is a "Server speaks first" protocol, so you need to structure your sideband to wait for the server banner.

I'm not entirely sure how you can handle the state if the sideband connection does not behave the same as the pool member, because that could make things very difficult to manage.