EC
554
SMTPERRORCommonDelivery FailureHIGH confidence

Transaction failed

What this means

The 554 code is a generic, permanent failure message indicating the transaction failed. It is often used when a message is rejected for policy reasons, such as failing a spam filter.

Why it happens
  1. 1The sender's IP address is on a blocklist (RBL).
  2. 2The email content triggered a spam filter.
  3. 3The recipient's server policy does not allow messages from the sender's domain (e.g., missing SPF/DKIM records).
How to reproduce

An automated system sends an email that is flagged as spam by the receiving server.

trigger — this will error
trigger — this will error
DATA
354 Go ahead
[...]
.
554 5.7.1 Service unavailable; Client host [x.x.x.x] blocked using zen.spamhaus.org

expected output

554 Transaction failed

Fix 1

Check IP Reputation

WHEN You operate the sending mail server

Check IP Reputation
// Use online tools to check if your IP is on any blacklists
checkBlacklists("your.server.ip.address");

Why this works

If your server's IP is on a blacklist, you must request delisting to ensure mail delivery.

Fix 2

Configure SPF and DKIM

WHEN You control the sending domain

Configure SPF and DKIM
// Add SPF and DKIM records to your domain's DNS settings
// SPF: "v=spf1 mx -all"
// DKIM: "v=DKIM1; k=rsa; p=..."

Why this works

These records prove that you are a legitimate sender for your domain, reducing the chances of being flagged as spam.

What not to do

✕ Immediately retry sending

A 554 is a hard bounce, often due to a reputation or policy issue. Retrying without fixing the underlying problem will fail and can further damage your sender reputation.

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