How to resolve SMTP 421 errors with Web.de and GMX.net due to rate limiting?
Matthew Whittaker
Co-founder & CTO, Suped
Published 4 Aug 2025
Updated 16 Aug 2025
8 min read
Encountering SMTP 421 errors when sending emails to Web.de and GMX.net users can be a frustrating experience. These error codes often indicate a temporary issue, but with German internet service providers (ISPs), 'temporary' can sometimes mean persistent rate limiting, even for low sending volumes. I've seen situations where email senders are stuck in a cycle of temporary failures, eventually leading to permanent rejections for being 'too old' or due to local processing errors.
The common message accompanying these 421 errors, such as "Nemesis ESMTP Service not available\nService unavailable\nReject due to policy restrictions," often directs you to information about rate limits being reached. While it suggests reducing the sending rate and trying again, even heavily throttled sending, like 5 emails per minute, might still face these rejections, especially when dealing with a new market or an unproven local mailbox provider (MBP) reputation.
This situation is particularly challenging if you're sending transactional or double opt-in messages, as these critical emails can also get caught in the rate limiting loop, impacting your entire user onboarding flow. While global reputation metrics from providers like Google and Microsoft might look solid, Web.de and GMX.net have specific stringent requirements that need to be met. So, what steps can you take beyond simply waiting for a response from their postmaster teams?
Understanding SMTP 421 and its implications for Web.de and GMX.net
SMTP error 421 is classified as a transient negative completion reply, meaning the mail server is temporarily unavailable to accept mail, but it expects to be able to do so later. The sending server should requeue the message and try again. However, Web.de and GMX.net often use this error specifically to enforce rate limits when they detect an unusual volume or suspicious patterns from a sending IP or domain. You can learn more about this on CloudMailin's SMTP troubleshooter page.
The key challenge with these German providers is that their rate limiting isn't always about the raw volume you're sending, but rather about the context and perceived reputation of the sending IP and domain, especially in a new market. Even small, carefully throttled sends can trigger these responses if the initial trust isn't established. This is similar to other rate limiting issues you might encounter.
If you're using a shared IP pool with an Email Service Provider (ESP) like Mailgun, this adds another layer of complexity. The rate limits are typically applied at the IP level, meaning the sending behavior of other users on your shared IP can negatively impact your deliverability, regardless of your own careful sending practices. This makes it difficult to ascertain the exact cause, as you don't have full visibility into the combined sending volume or the reputation of the entire shared IP pool.
Understanding 421 errors
Temporary state: An SMTP 421 error indicates a temporary problem. The sending server should typically requeue the message for a later retry. This is how the protocol is designed.
Rate limiting: For Web.de and GMX.net, this specific error often signals that you've hit their hourly or daily sending limits from a particular IP address or domain. It's a soft block (or blacklist) to prevent potential abuse.
Reputation driven: These providers are highly sensitive to sender reputation. If you're new to the German market or have a new IP, they might be extra cautious, even with low sending volumes.
Meeting technical expectations
Web.de and GMX.net are known for their strict adherence to email authentication standards and technical configurations. Beyond just SPF, DKIM, and DMARC passing, they often require a high degree of DMARC alignment, sometimes even blocking emails if alignment isn't perfect, regardless of a DMARC record's policy. This highlights the importance of strong email authentication.
One often-overlooked technical detail that German ISPs are particularly picky about is reverse DNS (rDNS). A misconfigured or generic rDNS record, especially on shared IP addresses, can lead to increased scrutiny and potential blocking. This is because rDNS provides a way to verify that the sending IP address is legitimately associated with the sending domain, adding another layer of trust.
Ensure your emails include a valid 'Date' header. While this might seem basic, a missing or improperly formatted 'Date' header can be interpreted as a sign of spamming by some mailbox providers, leading to rejections or being flagged as unsolicited mail. Double-checking these small technical details can significantly impact your deliverability.
They are particular about rDNS configuration. If on a shared IP, ensure your ESP's rDNS is clean and points correctly.
SPF and DKIM
While standard, ensure they are perfectly configured and pass all checks. Failures here will almost certainly lead to blocks.
Date header
A valid 'Date' header is crucial. Its absence or malformation can flag your emails as spam.
Strategic adjustments and warm-up
When facing persistent 421 errors, especially with new markets or low volumes, the fastest resolution is often to pause sending to these domains temporarily. This allows the perceived sending rate to reset and gives you a clean slate to begin a proper warm-up. Even if your initial volume wasn't high, the recurrent temporary failures and re-tries by your server can contribute to the perceived rate, making the situation worse. Remember that for ISPs, every connection attempt counts towards your rate limit, regardless if it's a first send or a retry.
Re-warming your sending reputation, even for domains that previously had good standing, is crucial when you encounter such issues. Start with a very small trickle of emails and gradually increase the volume over several days or weeks. This signals to the receiving ISP that you are a legitimate sender building a positive reputation. For more insights on this, consider our guide on IP warm-up strategies.
While double opt-in is a deliverability best practice, it's not a silver bullet, especially for new lists or markets. The age of your double opt-ins can sometimes play a role. If a list was recently collected, but the sending IP or domain is new to the German market, the ISP still needs to build trust. Maintaining a clean list and regularly removing inactive subscribers can also help in the long run by ensuring you're only sending to engaged recipients, which positively impacts your sender reputation.
Common sending pitfalls
Ignoring 421 errors: Assuming they will resolve themselves with automatic retries, without adjusting sending behavior.
Over-reliance on shared IPs: Not accounting for other senders' impact on your IP's reputation with German ISPs.
Neglecting technical nuances: Focusing only on volume, not on specific DNS configurations or header requirements.
Effective strategies
Strategic pause and re-warm: Temporarily halt sending to problematic domains and restart with a slow, gradual ramp-up.
Proactive ESP engagement: Work with your ESP to understand shared IP performance and request rotations if necessary.
Comprehensive technical review: Ensure all DNS records, especially rDNS, are correctly configured and aligned.
Resolving the issue and proactive measures
Beyond technical configurations, maintaining open communication with the postmaster teams at Web.de and GMX.net is important. While the process can be slow, providing them with details about your sending practices, list acquisition methods (e.g., double opt-in), and the steps you're taking to resolve the issue can build trust. They have postmaster pages that outline their policies and provide contact information.
If you're on a shared IP from an ESP, contacting your ESP is crucial. Shared IPs are a common source of unexpected rate limiting or being put on a blacklist (or blocklist). Ask them to investigate the IP's reputation with Web.de and GMX.net specifically. In some cases, your ESP might be able to rotate the problematic IP addresses out of your pool. This can offer immediate relief, though as ESPs often state, there's no guarantee the issue won't recur on other shared IPs.
For senders with growing volume to Germany, or those for whom deliverability to Web.de and GMX.net is mission-critical, considering a dedicated IP address becomes a strong recommendation. A dedicated IP gives you full control over your sending reputation, isolating you from the sending behavior of others. While it requires its own careful warm-up and monitoring, it offers the best long-term solution for consistent deliverability, especially with ISPs that have strict IP-based policies.
Proactive monitoring of your IP and domain reputation is key to preventing future issues. Tools that offer blocklist checks and DMARC monitoring can help you spot potential problems early, before they escalate into significant deliverability disruptions. Being vigilant about your sending metrics allows for swift intervention.
Action plan for resolving 421 errors
Contact ESP immediately: Inform them about 421 errors to Web.de/GMX.net. Request IP pool rotation if on shared IPs.
Pause sending to affected domains: Temporarily stop sending to Web.de and GMX.net addresses to reset their rate limits.
Re-warm slowly: Once issues subside, start with a very low volume and gradually increase. Patience is key.
Consider dedicated IP: For critical deliverability, a dedicated IP provides more control over your sending reputation.
Views from the trenches
Best practices
Actively manage your sending volume and gradually increase it, especially for new target markets.
Regularly review your DNS settings, including SPF, DKIM, DMARC, and especially rDNS, for perfection.
Engage proactively with your ESP regarding IP reputation and possible rotations if you are on shared IPs.
Maintain a pristine mailing list with recent double opt-ins and remove inactive subscribers regularly.
Common pitfalls
Assuming 421 temporary errors will self-resolve without changes to sending strategy or volume.
Underestimating the strictness of German ISPs regarding technical email authentication standards.
Failing to consider the impact of other senders on a shared IP address when experiencing rate limiting.
Not having a clear strategy for re-warming an IP or domain after a period of deliverability issues.
Expert tips
Check for any missing or incorrectly formatted email headers, as these can trigger blocks.
Understand that GMX.net and Web.de are very particular about DMARC alignment, even without an enforced DMARC policy.
If using a shared IP from an ESP, be aware that other senders on that IP can negatively affect your deliverability.
Be prepared to stop sending to problematic domains and re-warm them slowly, as this is often the fastest way to reset.
Expert view
Expert from Email Geeks says one German company is particularly picky about authentication aligning, and they are also very particular about technical aspects like rDNS. They previously blocked an ESP for rDNS issues.
May 9, 2022 - Email Geeks
Expert view
Expert from Email Geeks says that stopping sending and then re-warming is likely the fastest way to resolve persistent 421 errors with Web.de and GMX.net.
May 9, 2022 - Email Geeks
Navigating deliverability challenges with German ISPs
Resolving SMTP 421 errors with Web.de and GMX.net due to rate limiting requires a multi-faceted approach, focusing on both technical precision and strategic sending adjustments. While these are temporary errors, their persistence, especially with shared IP addresses and new market entries, can significantly impact your deliverability. German ISPs prioritize sender reputation and technical compliance, often more rigorously than others.
By pausing sending, strategically re-warming your IP or domain, ensuring impeccable technical configurations (especially DMARC alignment and rDNS), and maintaining proactive communication with your ESP and the ISPs, you can navigate these challenges. For mission-critical sending to Germany, a dedicated IP might be the most reliable long-term solution. Remember, consistency in good sending practices is what ultimately builds and maintains a positive sending reputation.
Google
and Microsoft might look solid, Web.de and GMX.net have specific stringent requirements that need to be met. So, what steps can you take beyond simply waiting for a response from their postmaster teams?
