I think that some of the servers respond to large (for some value of "large") numbers of e-mails being sent through them, by deliberately slowing down the rate at which the messages are sent. The theory is that for a legitimate run of a few dozen or a few hundred, it isn't going to make that much difference - the delay isn't going to do any harm. To a spammer, it will make the connection almost useless. You may be running into a situation like this.