How to Split Email API Responsibilities Between Your App and a Mail Provider

When building a customer-support email system, the key architectural decision is determining which responsibilities belong to the mail provider versus the application itself. Providers like Resend, Postmark, Mailgun, and Amazon SES each handle template rendering and suppression lists, while the application should own recipient eligibility, report generation, and attachment validation. Three core rules govern this boundary: suppressed recipients must be filtered before sending, attachments must be pre-generated and validated rather than fetched via live URLs, and every retry must carry the same idempotency key to represent a single logical operation. Violating the attachment rule risks two retry attempts referencing different document versions, making audit trails unreliable. Amazon SES suits teams prioritizing cost efficiency who are willing to handle more integration work, while other providers offer more built-in convenience at the expense of greater vendor dependency.