My first step would be to contact the ISP and see whether they are willing (perhaps for a fee), to solve this 'problem' at the MTA level, by using a throttle. That's a far more flexible solution, that avoids clutter in your programs as well. It also automatically solve the problem of having two programs simultaniously trying to set a large batch mails. You also don't have to reconfigure your program(s) if you and your ISP agree on a different limit of mails/hour.

It's an MTA level problem, and that's where it needs to be solved.

Abigail