Mailjet

Mailjet handles both marketing campaigns and transactional email through a unified API, which appeals to B2B development teams. It's part of the Sinch family and includes collaborative features designed for larger teams who need approvals and handoffs across multiple people.

The platform works well if you're sending volume email and need reliability. Many B2B SaaS companies use it for customer-facing transactional emails and marketing campaigns from a single system.