Mass Email User Limit

Hi there,

The plugin notes page says this plugin is suitable for sites with fewer than 5000 users, but upon install, says that only 2500 users are acceptable.

I know enough to dig around in the code and change that to a number I can live with, but:

1 - I thought someone would like to be aware of the discrepancy and
2 - I was hoping someone could explain the reasoning, or the formula used to arrive at that number.