here are three options can make the sending speed more fast:
1. sending thread. choose the suitable setting thread.
2. Domain settings. suitably increase "Max continuous connections" and
suitably decrease "Delay for continuing" time.
This option is aimed at the directly sending.
3. Relay Server. This option is aimed at the sending
via a relay smtp server. suitably change it configuration.

More important thing is revising the second and third options, they will
make more effects to the successful rate of sending.