THE PRICE OF RELIABILITY IS THE PURSUIT OF THE UTMOST SIMPLICITY.
Another case of doing manual labour over automation.
But was that really necessary? Certainly, an API link would be ideal. New users in our own system would automatically end up in the MailChimp subscriber list. And vice versa, those who would have unsubscribed could be marked in our own system.
But building the API would have taken at least a few man-weeks. Luckily, there's a much simpler solution. At that stage, the newsletter only had about one hundred subscribers. And the newsletter had a low frequency of once every month. Doing it manually was the simplest solution and it could be implemented within an hour. Just export the data from the database and upload it to MailChimp. Those who unsubscribed needed to be manually marked in our own system. Done!
The general lesson is here: don't build solutions for non-existing problems. Create luxury problems instead, which means: attract so many subscribers that an API is desirable. The API would be great when having ten thousand subscribers, but not just one hundred.