Need technical advice/thoughts on being an email service provider

So, our core competency isn't being an ESP. But as our product has grown, we've started to let our users send email blasts through our product. Our core product is a website builder, where our users can host a website with their custom domain.

Recently, we rolled out an email blast and email drip feature and built it on top of Sendgrid (which we're also using for our own transactional emails)

Question to other founders/devs that has similar functionality in their products.
Are you using services like Sendgrid, or SES, or even rolling our own email server? I know the answer is pretty involved, but I wanted to get an idea of high level server stack.

Things we want to take into account is deliverability and isolating any potential abusers who overly spam, and make sure they don't ruin our reputation with our ESP (Sendgrid).