Hi Jack,
I apologize for the delay - I've been overwhelmed but things are returning to normal.
Once ListMail sends messages to the server it is up to your web host to deliver the messages. If you are on a shared server (ie. typical web hosting) there is the possibility that another user on the server has been sending more email than your server can handle.
A server can send 100k+ per day so your 800 messages should have very little to do with it. Please talk to your host about the delays - they should get an administrator to take a look at the server.
Regards