Deliverability and authentication
Picking a great provider is necessary but not sufficient. Inbox placement depends on authentication (SPF, DKIM, DMARC), warming, suppressions, and engagement. Google and Yahoo's February 2024 bulk-sender requirements made authentication mandatory for any sender pushing more than 5,000 messages a day to either provider.
SPF · DKIM · DMARC · BIMI
Reputation and IP management
IP warming
Gradually ramping volume on a new IP to build reputation.
Dedicated IP
A sending IP reserved for one customer.
Shared IP
A sending IP used by many customers of a provider.
Suppressions
A list of recipients the provider refuses to send to.
Sandbox mode
A starter mode that limits sending to verified addresses.
Pre-launch deliverability checklist
- 01 Publish SPF for the sending domain. Validate with mxtoolbox or a provider tool.
- 02 Enable DKIM signing on the provider and add the public key to DNS.
- 03 Publish DMARC at p=none with an rua report address you control.
- 04 Verify the record in production by sending to a Gmail and Outlook address you can inspect headers on.
- 05 After two to four weeks of clean DMARC reports, tighten the policy to p=quarantine, then p=reject.
- 06 Send only to recipients who consented. Honor unsubscribe requests within 24 hours per CAN-SPAM.
- 07 Suppress hard bounces immediately. Most providers do this for you; verify it is on.
- 08 For volumes above 5,000/day to Gmail or Yahoo, confirm one-click List-Unsubscribe is configured.
reading this as teams choosing a transactional API
Judged purely as an API. What the request contract guarantees on retry, how complete the webhook event coverage is, whether suppressions are readable and writable programmatically, and how much the message log retains when something needs debugging in production.
Applied to this page, that means weighing idempotency keys, webhook coverage, event stream, and operating track record ahead of the rest, against high-frequency programmatic sends where a missed delivery is a product failure.