Diagnosing an SMTP deferral (4.7.0)
A deferral is a temporary delivery failure. The receiver didn't reject your mail, it asked you to try again later. 4.7.0 is one of the most common, and it usually means a policy or reputation issue. Here's how to read it and what to do.
Temporary vs permanent
SMTP results come with a reply code and an enhanced status code (class.subject.detail, per RFC 3463). The first digit is what matters most:
4.x.x(and4yyreply codes): a temporary failure. The message is valid but couldn't be delivered right now; retrying later may succeed. This is a deferral.5.x.x(and5yy): a permanent failure (a hard bounce). Retrying won't help.
BlacklistGuard automatically retries deferred messages over time, so a deferral isn't a lost message, it's a delay. But sustained deferrals mean something needs fixing.
What 4.7.0 means
In RFC 3463, the x.7.x subject is the security/policy class, and 4.7.0 is "other or undefined security status", a deliberately vague code receivers use when something about your sending triggered a policy block but they don't want to spell out exactly why. In practice, a 4.7.0 deferral almost always points to reputation or rate. For example, Yahoo returns 421 4.7.0 [TS01] ... unusual traffic patterns from your server when it's rate-limiting an IP.
Common causes
- A new or cold IP. Providers rate-limit IPs they don't recognize. Deferrals during IP warm-up are normal. Keep the ramp gentle and they ease off.
- Sending too fast. A burst to one provider trips rate limits. Add throttles to pace it.
- Reputation slipping. Rising complaints or bounces lower your standing and invite deferrals. Check your complaint rate and list hygiene.
- Authentication gaps. Missing or misaligned SPF/DKIM/DMARC can trigger policy deferrals.
How to diagnose it on BlacklistGuard
- Open Messages in the console (or the messages API) and find the affected messages. Each row shows the
status, the SMTP result code, and the fullerror_messagethe receiver returned. - Read the receiver's text. It often includes a code like
[TS01]or a link to the provider's postmaster page explaining the specific reason. - Check whether it's one provider or many. One provider usually means a reputation/rate issue with that provider; many at once points to something on your side (authentication, a bad send, a cold IP).
- Look at the trend on the Deliverability report. Are deferrals rising with volume or complaints?
What to do
- If it's a cold IP or a burst: slow down. Tighten throttles and let the IP warm. The deferrals are the provider asking you to pace yourself.
- If complaints/bounces are up: clean your list, validate risky addresses, and suppress complainers.
- If it names a reason: follow the link in the error to the provider's postmaster guidance. Many include a form to request a review of your IP.