Ask HN: An API to schedule emails much later?

On my website, I let users schedule reminders up to 3 years in the future. These are reminders that you are eligible for a pension refund 2 years after leaving Germany. Each email is a few fields rendered into a simple template.

I was surprised that none of the email delivery API let you schedule an email more than a few weeks ahead. I had to write my own, but I'd rather outsource this to a service.

Is there such an API? If not, why not?