Last updated: July 2026

In short: IONOS centrally blocks outbound port 25 across all Cloud Servers, VPS and Dedicated Servers by default. To request an unblock, your server must meet three technical prerequisites (FQDN, reverse DNS, SPF), and you must contact customer service directly by phone.

When running a self-hosted email infrastructure on an IONOS Cloud Server or Dedicated Server, outbound connections on port 25 will be intercepted by default. IONOS enforces this central network restriction across its hosting infrastructure to mitigate outgoing spam and preserve IP subnet reputation.

Scope and central network enforcement

The outbound port block applies across core IONOS server lines. According to the provider’s official documentation, the restriction is: Valid for Cloud Servers, VPS, VPS+, Dedicated Servers, and Server Power Deals managed in the Cloud Panel.

Regarding the security rationale and unblocking possibility, IONOS states: For security reasons, port 25 (SMTP) is blocked by default. If you want to send emails via your server, we can unblock this port for you.

Crucially, this restriction is enforced at the data center network level, rather than through local server or Cloud Panel firewall rules. IONOS clarifies: You do not need to make any changes to the firewall in the Cloud Panel for this activation, as the block is controlled centrally via our network infrastructure.

Three technical prerequisites for unblocking

Before IONOS customer service will approve an unblock request for port 25, your server configuration must satisfy three strict technical requirements:

  1. Valid FQDN: Your server must be accessible under a static, fully qualified domain name: Your server requires a valid, fixed domain name (fully qualified domain name, or FQDN for short).
  2. Reverse DNS (PTR) record: The server’s IP address must resolve exactly to your configured FQDN via reverse lookup.
  3. SPF record: Your sending domain’s DNS must publish a valid Sender Policy Framework (SPF) record authorizing the server’s IP address.

The mandatory phone unblock procedure

Once your FQDN, reverse DNS, and SPF records are published and propagated globally, you must initiate verification via phone support: If the three requirements above are met, please contact IONOS customer service by phone.

Unlike self-service unblocks or automated support tickets, IONOS mandates a live telephone verification with support agents before removing network-level restrictions.

IP reputation reality check

Even after successfully unblocking port 25 over the phone, delivering mail directly from an IONOS server IP presents deliverability challenges. Because cloud server subnets are frequently targeted by abusers, hosting IP pools routinely appear on strict blocklists (DNSBLs) such as Spamhaus and UCEPROTECT.

If you deliver mail directly from your IONOS IP address, you must continuously monitor your IP deliverability across major receiving platforms, as strict providers like Gmail and Microsoft 365 penalize unaligned or poorly reputed hosting subnets.

Smarthost relaying via port 587

To bypass phone verification queues and avoid the deliverability risks of hosting IP ranges, you can route your outbound mail through an external smarthost relay over port 587 (Submission). Port 587 is not subject to the central port 25 block.

A standard Postfix configuration (/etc/postfix/main.cf) for relaying via an external smarthost looks like this:

relayhost = [smtp.provider.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = encrypt

Or send reliably via Dispatch. If you want to avoid phone queues, technical prerequisite audits, IP reputation troubleshooting, and smarthost maintenance, you can route your outbound mail securely via Dispatch.

Verifying your configuration

Once your sending infrastructure is live, verify your email authentication records (SPF, DKIM, DMARC) and check your reverse DNS health using the free MXAudit scanner.

Further reading