You spin up a fresh VPS, install Postfix, and proudly send your first test email — only to watch it sit in the queue until it bounces. The culprit is almost always the same: the provider blocks outbound SMTP traffic. If you want to send mail directly from your server, you must unblock port 25 — and every provider has its own process for doing it. This guide walks you through exactly how to do it on the most popular VPS hosts, what to do when a request gets rejected, and the legitimate alternatives if unblocking is not possible.
Why Providers Block Port 25 by Default
Port 25 is the SMTP port used for server-to-server email delivery. Because spammers abuse it constantly, most cloud and VPS providers block outbound port 25 on new accounts. This keeps the provider's IP ranges off blacklists like Spamhaus — but it also means your mail server cannot deliver anything until you get the port opened. Note that some providers go further and block ports 465 and 587 too; always check all SMTP ports when testing.
Test Whether Port 25 Is Blocked on Your VPS
Before doing anything else, confirm the problem. SSH into your server and run:
telnet smtp.gmail.com 25 or nc -vz smtp.gmail.com 25
If the connection hangs or times out while ports 80 and 443 connect fine, outbound port 25 is blocked — either at the provider level, by your local firewall, or both. Check your firewall too (ufw status / iptables -L OUTPUT) so you don't chase the provider when the block is local. Also remember that a inbound port-25 block doesn't matter for sending; only outbound blocking stops delivery.
How to Unblock Port 25 on Popular Providers
The exact process varies by provider. Here is how the big ones handle it:
AWS EC2
AWS blocks port 25 on all EC2 instances by default. Open the EC2 console, create a support case under “EC2 instance port 25 throttle,” and describe your use case in detail: domain you will send from, expected daily volume, bounce-handling process, and that you have valid SPF, DKIM, and DMARC records. AWS approves most legitimate requests within 24–48 hours.
Hetzner
Hetzner's official policy is that outbound port 25 is blocked and generally cannot be unblocked on their cloud or dedicated servers. If direct mail delivery is your goal, Hetzner is simply the wrong provider — consider moving to one of the mail-friendly hosts listed below.
DigitalOcean and Vultr
Both block port 25 by default on new accounts. You can request removal by opening a support ticket and explaining your use case. Approvals depend on account age and trust: newer accounts are often denied, while established accounts with a clean billing history get approved. State your expected volume and confirm you will configure SPF, DKIM, and rDNS.
Mail-Friendly Providers That Make It Easy
A handful of providers cater to email senders and either ship with port 25 open or approve unblocking requests quickly. If you are choosing a VPS specifically for bulk or transactional mail, pick one from a list of providers that are known for mail-friendly policies — Contabo and RackNerd are two affordable options that bulk mailers use widely, and Ultahost is another VPS host with a reputation for fast support on mail-related requests. Our full comparison of the best VPS providers with open port 25 covers the trade-offs in detail.
What to Write in Your Unblock Request (Template)
Providers approve requests that look legitimate and deny vague ones. Include:
- Your use case: “transactional email for my application” or “newsletter for my subscriber list”
- Your domain and confirmation that SPF, DKIM, and DMARC are configured
- Expected daily/weekly sending volume
- How you handle bounces and complaints (e.g., list hygiene, feedback loops)
- Confirmation you will not send unsolicited email
Being specific here is the difference between approval in a day and a rejection you have to appeal.
After the Provider Unblocks Port 25
Getting the block lifted is only step one. Before sending real mail, complete these:
- Set reverse DNS (PTR) for your server's IP to match your mail hostname — Gmail and Outlook increasingly reject mail without it.
- Publish SPF, DKIM, and DMARC records for your sending domain.
- Warm up the IP — start with low volume and ramp up over 2–3 weeks. A brand-new IP blasting 50k emails on day one will land on blacklists fast.
- Monitor blacklists with tools like MXToolbox's blacklist check during the first month.
Once your mail server is installed — for example, following our guide on how to install PowerMTA on Ubuntu 24 — deliver a test to a Gmail and a Microsoft address and check the headers for SPF, DKIM, and DMARC pass results.
Alternatives When You Can't Unblock Port 25
Sometimes a request to unblock port 25 just isn't an option — Hetzner won't budge, or your account is too new to be trusted. In that case:
- Use an SMTP relay (smart host): route all outbound mail through a relay service like Amazon SES, SendGrid, or Mailgun on port 587. Your MTA still queues and signs mail locally; the relay handles delivery.
- Move providers: if the whole point of the VPS is email, migrating to a mail-friendly host is cheaper in the long run than fighting support.
- Keep port 25 blocked but use submission ports: for contact forms and application mail, port 587 via a relay is all you need — no direct MX delivery required.
Key Takeaways
- Test first: a hanging connection to smtp.gmail.com:25 confirms the block — rule out your local firewall before contacting support.
- To unblock port 25, open a provider support ticket with a detailed, legitimate use case including SPF/DKIM/DMARC status and expected volume.
- Policies differ wildly: AWS approves most requests, DigitalOcean/Vultr depend on account trust, and Hetzner will not unblock port 25 at all.
- Choose a mail-friendly VPS from the start if email is your core use case — it saves you the whole request process.
- After unblocking, set reverse DNS, publish SPF/DKIM/DMARC, warm up the IP gradually, and monitor blacklists.
- If unblocking fails, an SMTP relay on port 587 is a fully legitimate alternative.
Related Guides
- Best VPS Providers With Open Port 25 — the full comparison of mail-friendly hosts.
- How to Install PowerMTA on Ubuntu 24 — set up your high-performance mail server once the port is open.
- How to Install Zimbra Mail Server on CentOS 7 — a full mail suite option for self-hosters.