But if you have millions of users, or even 5000. Or in my case, 19,600 who desire to receive your email. If the browser closes on accident, or there is a power cut due to thunder, etc .. And you're already at 15,000 then restoring a session of the browser won't help. It's a POST input only accepting state thingymebob. Result: you're screwed. In my case 4,500 didn't receive their mail, unless I accept the risk of having to re-mail the mail to the first 15,000.
It would be nice if the mail simply has a 'state' and a table where it stores the submitted mail.
So when you come back at any point it says: 'mail is in progress, 15,000 out of 19,600 [attempt to continue] [close mail job].