Don't support yields per x lines on SMTP receive

Consume the SMTP lines as fast as possible since the sender may be on the
local network (or even local/loopback interface!) and could get way ahead and
timeout otherwise, resulting in the "premature evacuation" error (and dumping
of the received message) on the receive side. This resolves an observed issue
with sending large attachments to the mail server at very high rates and the
sending client timing and disconnecting waiting for a response from the server
(which was throttling the receive using YIELD).