What do services like ConvertKit and MailChimp use to send their email?

Hey Ben 👋

Dunno about Convertkit, but Mailchimp has created his own infrastructure and stack for emails, so they are not relying on any 3rd party APIs for the sending.
It's kinda their main offering and they are the reference in the field, so the opposite would be daring 😄 (imagine the provider is not reliable or worse, stop working?)

Regarding your questions, it's tricky, so let's think a bit.

If you're just starting in the field, or it's not your main business area, it can make sense to use a 3rd party API (for example the one made by Mailchimp) to send the emails. However, to actually sell that to a lot of customers, you'd need to provide things that Mailchimp does not on top of that API so it's actually an advantage to go through the new service than through the existing one.
As an example, I did that 8 years ago when I was working on Kimple and we were using Mailchimp and Mailgun's API to send the emails, and we were just providing a all-in-one tools for brands to handle their marketing contest and do reactivation through emails. It was simpler for them to use the service we provided and they didn't mind spending more bucks to just remove the learning curve, so it was fine.

In other cases, you can possibly try to abstract the 3rd party API so it's easiest to use or it covers specific use case like in no-code solutions, but it's unlikely it's going to become the reference in email sending, as sooner or later, people will know you rely on external sending services, and just turn over to them.

So, it's technically possible, can be an avantage sometimes, can be profitable if you focus on very specific niche that don't care about you using third-party providers (i.e email sending is not your main activity), but it's unlikely you would be able to overcome competition and have stable income otherwise 😊