[Exim] Mail Trickling.

I have the following (legitimate) need:
To send large quantities of duplicated mails to individual addresses in
a given domain from time to time. (i.e. >50,000 users)

I was wondering what the best strategy was to implement this via a local
exim MTA.

Asuming the mails are sent at a low presidence (i.e Bulk)

Or, is this best done by trickling mail to the local smtp server?

Thanks,
Owen Cliffe.