Any thoughts on why when I send 10k emails my system stops at 4150, and then if I load the rest in and send again it stops at 7550? so then I have to load the rest and it finally finishes.

Zimbra 7.1, Debian 6.0,

swiftmailer with;
SMTP transport;
$mailer->registerPlugin(new Swift_Plugins_AntiFloodPlugin(50, 15));
a full second wait after each email sent and 2 second after each 25 sent.