Background
We're building an application that will process & send emails via Mailgun. These are sometimes one-off messages, initiated by a transaction. Some emails, though, will be sent to 30k+ at once.
Eg, a newsletter to all members.
Considerations
Mailgun offers a Batch Sending option with their API. Using "Recipient Variables", you can include dynamic values that are paired with a particular user.
This Batch Sending functionality is limited, however. You cannot send more than 1,000 recipients per request, which means we have to iterate through a recipient list (on our database) for each set of 1,000. Mailgun provides an example of how this might work, using Python (scroll about 2/3 down).
Question
Are there any advantages to batch sending (ie, sending an email to a group of recipients through a single API call, using recipient variables) as opposed to making our own loop, variable substitutions and individual API calls?
I assume this is more taxing on our server, as it would be processing each message itself, instead of just offloading all that data to Mailgun's server for heavy-lifting on their end. But I also like the flexibility & simplicity of handling that on our end and sending a "fully-rendered" message to Mailgun, one at a time, without having to iterate 1k at a time.
Any thoughts on best practices, or considerations we should take into account?