Postmark is the gold standard for transactional emails. They have the best deliverability since they don't mix marketing emails.
Postmark has been doing bulk emailing since 2019.
> Q: How will you maintain high deliverability for transactional email?
> A: Separating transactional traffic from other types of email using different sending IPs and From domains is a longstanding best practice for ensuring optimal deliverability.
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> That means transactional and broadcast traffic never intersect in Postmark, including IP ranges.
https://postmarkapp.com/blog/api-bulk-the-final-frontier#how...