You'll have to disable "PIPELINING" in the ehlo response.
We could still be stalling the connection on the HELO/EHLO and DATA commands if PIPELINING is enabled (or that is what I assume from http://tools.ietf.org/html/rfc1854).
On the other hand, has anybody tried to put the "earlytalking" logic into the tarpit plugin? Any results?
