Last updated: July 2026

In short: Mail servers operated by GMX and WEB.DE (Nemesis ESMTP Service) rely on standard 3-digit SMTP status codes like 550, 554, or 421 without Enhanced Status Codes, accompanied by a specific diagnostic URI. Error 554 indicates an IP blocklist restriction, whereas status 421 warrants automated server retries.

When delivering email to recipients using GMX.net or WEB.DE, you interact with the shared email platform operated by United Internet (1&1 Mail & Media) under the banner Nemesis ESMTP Service. Managing tens of millions of consumer mailboxes across Europe, this infrastructure enforces strict inbound edge filters to defend against automated spam blasts and abuse.

Error Message Structure at GMX and WEB.DE

Unlike many major receiving platforms, GMX and WEB.DE explicitly omit Enhanced Status Codes (such as 5.1.1 or 5.7.1) from their rejection strings. According to official postmaster documentation, the exact structure is standardized: Error messages issued by our mail servers consist of: SMTP status code (e.g. 550 , 421 , but we do not use any additional Enhanced Status Codes , e.g. 5.1.1 ). Description of the problem. URI , that leads to a postmaster page specific to the problem.

Every bounce response delivered during the SMTP session includes a specific web link (URI) at the end of the line, directing administrators to the precise explanation and contact form for that specific delivery failure.

Error 554 — IP Address is Block Listed

Exact error string from Nemesis ESMTP Service

554-gmx.net (mxgmx104) Nemesis ESMTP Service not available 554-No SMTP service 554-IP address is block listed.

Meaning and root cause

Your mail server’s public IP address has been added to the internal blocklist maintained by 1&1 Mail & Media. Common causes include recent spam trap hits from your IP, missing or broken reverse DNS (PTR) records, or appearing on external DNS blocklists monitored by GMX and WEB.DE.

Concrete fix steps

  1. Follow the specific URI: Always open the exact web URL (URI) embedded directly within your 554 rejection string. That link directs you to the specific postmaster troubleshooting page and delisting form corresponding to your exact block scenario.
  2. Verify reverse DNS and authentication: Confirm that your sending IP has a valid, forward-confirmed reverse DNS (PTR) record and that your domain publishes compliant SPF and DKIM records.
  3. Submit a delisting request: Use the official GMX or WEB.DE postmaster contact form to submit a delisting application (IP unblocking request) once all underlying server issues are resolved.

Realistic timeline

The postmaster security team reviews incoming unblocking applications manually. After verifying that spam activity has ceased and technical requirements are met, IP unblocking typically completes within 24 to 48 hours.

Error 421 — Temporary Throttling and Recommended Retries

How to handle temporary rejections

When your mail server encounters a 421 rejection, the receiving server is enforcing a temporary rate limit—often triggered by opening too many concurrent connections or exceeding volume thresholds within a short window.

Official postmaster guidance outlines the exact protocol behavior: These errors are temporary. Subsequent delivery may be successful. In such cases, we recommend automated delivery attempts (retries) by the sending mail server.

Concrete fix steps

  1. Do not intervene manually: Do not cancel queued messages or submit delisting forms for status 421 rejections.
  2. Allow automated retries: Configure your mail transport agent (MTA, such as Postfix or Exchange) to perform standard, exponential delivery retries without aggressive bursting.
  3. Smooth concurrency limits: If status 421 occurs frequently, lower your server’s maximum concurrent connection limits (concurrency) specifically for destination hosts gmx.net and web.de.

Realistic timeline

Temporary rate limitations (421) clear automatically as connection volume subsides. Queued messages are typically delivered successfully during subsequent retry cycles within 1 to 6 hours.

Verifying your configuration

To confirm that your mail server settings and domain authentication records (SPF, DKIM, DMARC, and PTR) meet the technical standards required by Nemesis ESMTP Service, audit your domain instantly using the free MXAudit scanner.

Further reading