Last updated: July 2026

In short: Mail servers hosted by IONOS (formerly 1&1) reject outbound messages when sender policies are violated (550 Reject due to policy restrictions), when too many non-existent recipient addresses are contacted, or when the HELO/EHLO hostname rotates dynamically. Review exact postmaster definitions and fix workflows below.

When delivering mail to recipients whose mailboxes are hosted on IONOS systems, your mail server encounters a multi-layered security infrastructure. To protect consumers against phishing and spam, IONOS rigorously evaluates both technical domain authentication and connection-level MTA behavior.

Error 550 — Reject due to policy restrictions

Exact error string from IONOS

550 Reject due to policy restrictions

Meaning and root cause

This permanent rejection (550 hard bounce) indicates that your email failed technical compliance checks at the IONOS inbound edge. Common triggers include:

  • Missing authentication: Your domain lacks a valid Sender Policy Framework (SPF) record or a valid DKIM signature, or DMARC alignment checks failed.
  • Dynamic IP range: Your mail server is attempting to deliver mail directly from an unauthenticated residential or dynamic ISP IP address.
  • Broken reverse DNS: The reverse DNS (PTR) record of your sending IP address is missing or does not align with the forward hostname reported during connection handshake.

Concrete fix steps

  1. Verify SPF, DKIM, and PTR: Ensure your public IP address has a forward-confirmed reverse DNS record published and that your domain maintains valid SPF and DKIM entries.
  2. Use an authenticated relay: If you host an email server on a residential connection or dynamic IP range, configure an authenticated smarthost relay through your provider.

Realistic timeline

Once your domain authentication records and reverse DNS entries propagate across global DNS resolvers (typically requiring 15 to 60 minutes), IONOS edge relays resume mail acceptance without restriction.

IP Block Due to Elevated Bounce Rate

Exact guidance from postmaster documentation

Due to a poor delivery rate by sending to too many non-existent email addresses, the IP address of your email server is blocked from sending email to IONOS customers.

Meaning and root cause

IONOS anti-abuse filters track the proportion of delivery attempts that result in recipient errors (550 Requested action not taken: Mailbox unavailable). If a mail server attempts to deliver messages to excessive non-existent mailboxes in a short timeframe (such as during directory harvest attacks, compromised script broadcasts, or uncleaned mailing list campaigns), IONOS blocks the sending IP address across its entire customer base.

Concrete fix steps

  1. Purge invalid recipients: Immediately prune your subscriber lists and database records to remove any email addresses that have previously returned hard bounces (550 Mailbox unavailable).
  2. Enforce confirmed opt-in: Implement double opt-in procedures on registration forms to prevent invalid, mistyped, or malicious addresses from entering your distribution queues.
  3. Submit a delisting request: Once your subscriber lists are sanitized and server security is verified, submit a formal IP unblocking request through the official IONOS postmaster contact portal.

Realistic timeline

The IONOS postmaster team evaluates IP unblocking applications manually and via automated reputation checks. After verification, delisting is typically completed within 24 to 48 hours.

HELO/EHLO Throttling (Temporary Delay)

Exact guidance from postmaster documentation

The delivery of emails was temporarily delayed due to a constantly changing HELO/EHLO entry of your email server.

Meaning and root cause

During the initial SMTP connection (HELO or EHLO command), if your mail server dynamically rotates its reported hostname across successive connections or generates randomized identifier strings, IONOS identifies this as botnet activity. In response, the server enforces temporary throttling (greylisting/delays) to slow down delivery attempts.

Concrete fix steps

  1. Configure a static FQDN: Set your mail transport agent (MTA, such as Postfix myhostname or Exchange Send Connector configuration) to report one single, static, fully qualified domain name (mail.yourdomain.com) during every connection.
  2. Align with reverse DNS: Ensure that your static HELO/EHLO hostname matches the reverse DNS (PTR) record assigned to your sending IP address.

Realistic timeline

Once your MTA is configured with a consistent, static HELO/EHLO hostname and demonstrates stable connection behavior, IONOS automated throttling lifts automatically within 1 to 4 hours.

Verifying your configuration

To verify that your server configuration and domain authentication records (SPF, DKIM, DMARC, reverse DNS PTR, and HELO alignment) comply with IONOS security guidelines, audit your domain instantly using the free MXAudit scanner.

Further reading