That is certainly a good option - and talking things over with your ISP would give you the general idea about how many minutes there should be between the bundles.

An alternative would be to either set up your own MTA (which would send mail directly to addressees) or modifying your script to use Net::SMTP to do the same thing. The later is not trivial at all (since you will need to implement MX lookups, retries and non-deliveries), so I would still suggest you go with the first option.