My mails did not get to all my contact details and returned mails claim i have exceeded my maximum mails per hour.
My host accept 750 mails per hour and i set the newsletter plugin to 650 mails per hour.
I was surprised how i could exceed the maximum mails per hour with my settings.
The plugin does not send more than the configured emails per hour, have you other plugins sending a relevant number of emails, that, added to the ones sent by Newsletter, get over the limits?