421 mxgmxXXX Nemesis ESMTP Service not availableWhat does 421 Nemesis ESMTP Service not available mean?
Nemesis is the proprietary mail server software developed by 1&1 Ionos (United Internet AG) and used to handle inbound mail for GMX, WEB.DE, mail.com, freenet.de, and 1&1-hosted domains. When Nemesis returns a soft 421 with Service not available, it is throttling your sending IP based on velocity, volume, or temporary reputation degradation. This is distinct from the hard 554 Nemesis IP block listed: 421 is recoverable by slowing down. Nemesis aggressively throttles cold IPs, sudden volume spikes, and senders with degrading reputation signals.
Is this a soft or hard bounce?
Continuing to send at the same rate prolongs the throttle. Reduce velocity to the affected Ionos domains by 50 to 75 percent, then ramp back up over hours.
Common causes
Sending too fast triggers Nemesis throttling. Limits are not published but are stricter for cold IPs and during reputation evaluation.
Sustained traffic followed by a sudden 10x or 100x spike trips throttling regardless of reputation.
IP with neutral or borderline reputation receives stricter throttling than well-established senders.
How to fix it
Pause all GMX, WEB.DE, mail.com, freenet.de, 1and1.com sends. Restart at 25 percent of previous rate, smooth volume over hours.
High bounce rate to Ionos domains accelerates throttling. Use email verification to remove invalid German consumer addresses.
If 421 persists for over 24 hours of reduced sending, the throttle has escalated to a soft block. Treat as imminent hard listing and consider postmaster contact.
Provider-specific notes
| Domain | Operator notes |
|---|---|
| gmx.de, gmx.net, gmx.com | All routed through Nemesis. Same throttle, same recovery path. |
| web.de, freenet.de | Same Nemesis infrastructure. WEB.DE is the largest German consumer mail provider. |
| mail.com, 1and1.com, ionos.com | Global Ionos brand. Mail.com is consumer; 1and1.com / ionos.com are hosted business. |
Related error codes
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About the Author
Alaa · LinkedIn
Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. I’ve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.
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