1. Core Advantage: All Ports (Including Port 25) Open by Default
Most global cloud providers (Hetzner, DigitalOcean, Vultr, AWS EC2) block outbound TCP Port 25 by default to curb spam, demanding lengthy identity verification tickets that are frequently rejected.
🚀 Netcup Policy: Full Port Freedom Out of the Box
Netcup adheres to true infrastructure neutrality: every single VPS and Root Server instance comes with all network ports (including outbound TCP 25 for SMTP) completely open by default from day one. You never have to submit a ticket or wait for manual approvals to deploy Mailcow or Postal.
2. Verifying Outbound Port 25 Connectivity in Terminal
Verify outbound Port 25 connectivity with a single test command:
# Test direct connection to Gmail SMTP on Port 25
nc -zv smtp.gmail.com 25
# Immediate successful response:
Connection to smtp.gmail.com (142.250.xxx.xxx) 25 port [tcp/smtp] succeeded!
3. Configuring rDNS / PTR in Netcup SCP
While Port 25 is wide open, a matching reverse DNS (rDNS/PTR) is mandatory for deliverability. Without it, Google and Outlook reject deliveries. Set your PTR records in the SCP:
- Access the Server Control Panel: https://www.servercontrolpanel.de
- Select your server from the instance dashboard.
- Click "Network" on the left menu tab.
- Locate your IPv4 and IPv6 rows and click the "rDNS / PTR" edit icon.
- Set the reverse DNS to your mail server FQDN (e.g. mail.yourdomain.com) and save.
- DNS propagation typically takes 15 minutes to 1 hour across global recursive resolvers.
4. 10/10 Deliverability Authentication Checklist
| Record Type | Host | Example Value | Authentication Purpose |
|---|---|---|---|
| MX | @ | mail.yourdomain.com (Priority 10) | Points incoming mail to your Netcup mail server |
| TXT (SPF) | @ | v=spf1 mx ip4:YOUR_IP ~all | Declares that only your Netcup IP is authorized to send email |
| TXT (DKIM) | dkim._domainkey | v=DKIM1; k=rsa; p=MIIBIjANBgkqh... | Cryptographic signature verifying message integrity |
| TXT (DMARC) | _dmarc | v=DMARC1; p=quarantine; rua=mailto:[email protected] | Policy instruction for receiving mail providers if SPF/DKIM fails |
5. Maintaining Clean IP Reputation & AUP Compliance
🛡️ Responsible Sending Commitment:
Freedom comes with responsibility: Netcup strictly prohibits unsolicited commercial email (SPAM). Ensure your SMTP daemon is not configured as an open relay, as Spamhaus or Barracuda blacklisting triggers automated abuse termination.
6. Frequently Asked Questions (FAQ)
Q: Does a newly provisioned Netcup server truly not require unblocking Port 25?
Yes, absolutely! Unlike Hetzner, DigitalOcean, or AWS which strictly block port 25, Netcup keeps all outbound and inbound ports—including TCP Port 25—completely open by default. No support tickets or approval delays required.
Q: Why do outgoing emails bounce when sending to Gmail or Outlook?
The most common cause is missing reverse DNS (rDNS/PTR) or incomplete SPF/DKIM signing. Google and Microsoft enforce strict rDNS checks. Set your PTR record in Netcup SCP under the "Network" tab to match your mail hostname.
Q: Why is rDNS configured in SCP rather than in CCP domain DNS?
Reverse DNS (PTR) belongs to the IP block authority rather than domain zones. Because Netcup controls the public IP space, PTR records are managed in the Server Control Panel (SCP).
Markus S.
Senior Cloud Infrastructure Architect & Linux Sysadmin
Markus focuses on European cloud hosting economics, server performance optimization, and KVM virtualization. All benchmarks, setup guides, and VAT exemption procedures are verified on self-funded Netcup instances hosted in the Nuremberg datacenter (AMD EPYC hardware).
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