Mailgun

Mailgun occupies similar territory to SendGrid — a developer-focused email API for sending, tracking, and validating email at scale — but it's known for particularly strong email validation tooling, which matters for a newsletter list that's grown large enough to accumulate typos, dead addresses, and spam traps over time.

It's most useful if you or someone on your team is comfortable working with an API directly, since Mailgun doesn't offer much in the way of a writing or design interface. You'd typically pair it with a separate tool for actually composing issues, using Mailgun purely as the delivery layer underneath.