Where do you get the e-mails from? Mysql?
If yes, then i would suggest:
add a "lastsent" field, and each time you send a e-mail, update that field with a time().
So before you sent emails build a if($lastsent > something) {
do_send();
} else {
walk_away();
}
I don't know if I got you right, but i hope you got me!