Am I correct that the problem is the number of copies of the same email stacking up on the mail server, waiting to be sent? Obviously, slowing the rate you send emails at won't fix this, unless none of them have problems and the mail server sends faster than it receives. What you need is an intermediary utility that collects all emails and queues them only as fast as the mail server can handle sends. Perhaps you can add each email body to a database table along with the set of addresses it needs to go to (many to one relationship in second table?), then have an intermediary utility perform all the sends at some rate determined by how much the mail server can handle. This way you don't have the mail server clogged with thousands of copies of the same email while you wait for it to be sent, and you don't have multiple emails stacking on top of one another when different people want to send at the same time.