This program is only used about once a week, but the ISP is complaining about the volume of mail.

I manage the update mailings for a conference, and have a similar challenge, though at a larger (5x) scale. Our monthly emails aren't time critical, so having the mailing spread out over 24 hours, sent in batches of 25 emails several times an hour, works just fine. Sorting the mailing list alphabetically introduced enough variation to ensure that a single batch couldn't contain more than 5 emails bound for any single domain. This reduces the chance that some heavy-handed email filter will cry "spam", forcing me to jump through time-consuming hoops with some ISP so that the folks on or opt-in mailing list will continue to get their updates.