I've recently had the same problem but found a solution by using a PHP front end and a PERL arse.....
Create a table (name: mail_pending, columns: id,to,content,subject) and use a PHP front end to insert the mail messages into this table.....
Then, create a perl script that will select the messages from the table one-by-one until there are no rows left, use the perl sendmail function to send the messages...
Run this perl script from cron every 5 minutes. If there is no pending messages then the process will die, otherwises the process will build up and work together until all messages are sent.... (more processes....faster mailing!)
Hope i've helped....
Ed.