I wrote an custom multithreaded SMTP server with perl and used a program called postal to benchmark it.

I was able to get about 4,000 SMTP transactions per minute from a local network by 'pre-sporking' threads to sit and listen on sockets:

use strict;
use threads;
use IO::Socket;

# the port to listen on:
my $PORT = 8989;

# the number of threads to pre-spork:
my $INITHREADS = 5;

my $handle = IO::Socket::INET->new(
    LocalPort => $PORT,
    MultiHomed => 1,
    Type => SOCK_STREAM,
    Reuse => 1,
    Listen => 10) or die "Cannot open port $PORT: $!\n";
$handle->autoflush(1);

for(1..$INITHREADS)
    { my $thr = threads->new(\&process)->detach(); }

sleep(); # The parent thread sleeps forever. . .
# You can potentially change this perma-sleep to
# have the parent thread do dynamic thread management
# but only if you don't detach() them.

sub process {
    while(my $socklet = $handle->accept()) {
    # if you want to capture the IP address:
    # my $peerhost = $socklet->peerhost();
    # Here you can do your stuff
    # I have the server talk to the client
    # via print $socklet and while(<$socklet>)
    }
}
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