Why we built the SMTP Field Manual

ISPs (like Gmail, Office 365) will send responses back about the success or problems for the messages you send. Even in the industry, these responses are both misunderstood and not well documented. To make this easier, we built SMTP Field Manual to:

  • Provide a single resource for both documented and undocumented SMTP codes from the major providers.
  • Allow people to contribute to changes in SMTP codes.
  • Bring clarity on what actions people should take for common SMTP Codes (hard bounce, soft bounce, block, etc).
  • Bring these codes to the surface. Some ESPs (We won't name names ;) ) like to bury this information since it is telling about delivery.

We hope this becomes a valuable resource for anyone who relies on email delivery. Please help us spread the word to make it even more valuable.

Who it’s for

This resource is perfect for:

  • Customer Support – Make sense of that bounce message so you can tell your customer what happened, and how to fix it.
  • Email Server Administrators – Follow best practices to standardize SMTP responses across ISPs.
  • Email Deliverability Teams – Keep up to date on changes to SMTP responses, what they mean, and help contribute to the resource.

We email

Thanks to all the fine folks who have contributed to the SMTP Field Manual.