Here are some things to try:
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Use e3 Delivery Service to deliver your messages instead of the SMTP delivery method. There is no daily limit when using e3 Delivery Service.
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Direct Mail can throttle the number of messages it sends to your ISP. Choose Direct Mail > Preferences... and click "SMTP Accounts". At the bottom of the Preferences window click "Send no more than X messages at a time to SMTP server".