To throttle message delivery:

Choose Direct Mail > Preferences...

Click "SMTP Accounts".

Choose the SMTP account that you want to throttle.

Check the "Send no more than (20) messages at a time to SMTP server" check box and enter the maximum number of messages you want delivered per connection.

Optionally, check the "Wait in between connections" check box if you want Direct Mail to pause in between connections to your SMTP server.