Host Monster by default puts a limit on the amount of emails you can send per an hour.
The reason for this is so we don't have users using spam and slowing the servers down.
At first I thought this was bad but Host Monster is doing the right thing.
The limit is 50 emails per an hour.
Here is a tip, you can email them and get the amount lifted. I wrote a script to send about 300 emails per an hour and it works nicely.
Host Monster are always helpful and do the right thing by the customer
Give them a try you won't be disappointed