MailerSend

MailerSend is an API-first platform built specifically for agencies that need to send mail on behalf of many clients, each from their own domain. Every client gets separate domain management, separate analytics, and role-based access controls so you can let them view their own reports without seeing other brands' data. The infrastructure is optimized for deliverability, which matters when your reputation depends on getting through to inboxes consistently.

Perfect for agencies building marketing software, managing white-label platforms, or sending transactional and marketing emails across dozens of client accounts. If you're embedding email into a product or SaaS platform you're running for clients, the multi-tenant design handles the complexity of keeping each client's data and sending isolated. Strong API documentation and webhooks make it straightforward to build custom workflows, and you don't pay per contact or get locked into templates.