Sounds like you're on a workable track, but is there any reason not to batch them, with 5 or 10 addressees per email? Bcc if non-identification of other users is important, but batching, if I understand correctly, should reduce the workload on the server, at least to some extent, again, IIUC, especially is using SMTP rather than SENDMAIL..

And as to your timing issue: if your host (or admin if you own the server) sets limits, it should be easy to work out a value for $time if you use sleep($time).