We've been sending email (as an email service provider) for nearly 16 years at Wildbit. Arguably the most confusing aspects of email delivery is bounce management and SMTP response codes. If you're unfamiliar, ISPs (like Gmail, Office 365) will send SMTP responses back about the success or failure of the messages you send.
To bring more clarity and accuracy to SMTP responses, we just launched the SMTP Field Manual to be a one-stop resource for understanding all SMTP error codes from all the major inbox providers.
Want to give a try? Here are a few examples:
- SMTP Code 550
- iCloud 421 4.7.1 Message Deferred
- 550 5.7.1 Unauthenticated ... is not accepted due to domain's DMARC policy
We wanted to document these SMTP responses for several reasons:
- SMTP response codes vary wildly per provider. A 550 might mean something different depending on who you email.
- SMTP response codes are constantly changing. It's extremely hard to keep up with it.
- Advice on what to do with a specific response varies. Should this be a hard bounce, soft bounce, block?
- The information that comes from SMTP codes can be very telling for deliverability, yet most providers bury the information.
- Even ISPs themselves don't document all of these responses. A favorite is the Gmail low reputation response.
With these reasons in mind, we wanted to create a single resource to document the SMTP codes that exist for the major providers. And even more important, ask for help from the industry to keep them up to date.
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 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.