I read through this thread and didn't see any mention of this... sorry if I duplicate a suggested solution. I tried sending bulk mail to my 1000+ users and only 100 went out. Using the search bar on the Dashboard searched for "logs" and found the mail error log. There were my other 1000 or so emails that failed. Turned out, my host provider's email servers only allow 100 emails per hour to go out... after that you are shut off and additional emails are "errored out". I looked for a way to configure the Invision bulk mail to adhere to these rules but couldn't find anything. Went back to the error log and clicked the resend button 100 emails at a time... waited an hour and clicked 100 more (about a minute to click 100 times). Over the course of 3 or 4 days I clicked them all and looking at the returned mail in my SMTP account scored about 99.4% success rate and most of the fails were bad email addresses.