I need an email API that works for a platform with many customers and different sending domains. What should I use?

If you need an email API for a multi-tenant platform with many customers and different sending domains, you should choose an email provider that supports:

• Multiple sending domains / subdomains
• API-based sending
• Domain authentication: SPF, DKIM, and ideally DMARC support
• Per-customer isolation for stats, suppression lists, and reputation
• Webhooks for delivery/bounce/open events
• Role-based API keys or subaccounts if you want to separate customers cleanly

Good options

1. SendGrid

Best if you want a mature API and strong support for multi-domain setups.

Pros

• Easy API
• Supports multiple domains and subusers
• Good deliverability tooling
• Webhooks and event tracking
• Widely used for SaaS platforms

Cons

• Can get expensive
• Account management can be a bit cumbersome at scale

2. Postmark

Best if you care most about high deliverability and transactional email.

Pros

• Excellent deliverability reputation
• Simple API
• Great event webhooks
• Strong support for message streams / separation

Cons

• Less focused on large-scale multi-tenant marketing use cases
• Usually not the cheapest option

3. Mailgun

Good if you want flexibility and domain management.

Pros

• Strong domain-based sending
• Good API
• Useful for multi-tenant apps
• Event webhooks and suppression handling

Cons

• Deliverability and support experience can vary
• Pricing can add up with volume

4. Amazon SES

Best if you want low cost and are comfortable managing more yourself.

Pros

• Very cheap at scale
• Supports many verified domains
• Reliable infrastructure
• Works well in high-volume systems

Cons

• More setup and more operational work
• Less polished UX
• You need to manage deliverability and reputation more actively

5. SparkPost / Resend / Brevo

These can also work depending on your needs, but the four above are the most common choices for multi-domain SaaS setups.

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My practical recommendation

If you want the easiest production path:

SendGrid or Mailgun

If you want best transactional deliverability:

Postmark

If cost matters most and you can handle more ops:

Amazon SES

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What to look for specifically for your use case

For a platform with many customers, ask the provider whether they support:

• Per-customer sending domains
• Automatic DKIM/SPF setup
• Subaccounts or tenant-level separation
• Dedicated IPs or IP pools if customers need isolation
• Webhook routing by domain or tenant
• Bounce/complaint suppression per tenant
• Sandbox/testing mode
• Template support and dynamic variables

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Strong recommendation by scenario

• SaaS platform with many customers: SendGrid or Mailgun
• Transactional-first product: Postmark
• Need lowest cost at scale: Amazon SES
• Need per-tenant isolation and enterprise controls: SendGrid or SES with your own tenant abstraction

If you want, I can also give you a shortlist based on your exact requirements like:

• expected monthly volume
• transactional vs marketing
• need for dedicated IPs
• whether each customer must use their own domain