IONOS SMTP Error 550 Reject due to policy restrictions & HELO Throttling

Are your emails bouncing from IONOS mail servers with error 550 or HELO/EHLO delays? Discover exact postmaster definitions and fix steps.

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) is issued at the IONOS inbound edge. IONOS's postmaster documentation lists two causes for this specific code:

Concrete fix steps

  1. Check blocklists and request delisting: check with a blocklist provider such as Spamhaus whether your address is listed as a spam sender, and apply for a deletion of the entry.
  2. Set a valid HELO/EHLO: assign a fully qualified domain name (FQDN) to your mail system so it identifies itself correctly in the SMTP handshake.

Realistic timeline

Once any blocklist entry is removed and your mail server presents a valid FQDN in its HELO/EHLO, IONOS resumes accepting your mail. Blocklist delisting times depend on the provider — often a few hours to a couple of days.

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 Kuveris scanner.

Further reading