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Why VPS providers block port 25
Port 25 is the traditional server-to-server SMTP channel. A self-hosted mail server uses it to connect to the recipient domain’s MX server and deliver messages directly. Because compromised servers can use the same channel for spam, many VPS companies restrict outbound connections on that port.
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DigitalOcean’s current Droplet policy says: “SMTP ports 25, 465, and 587 are blocked on Droplets to prevent spam and other abuses on our platform.” AWS also blocks outbound port 25 by default on Lightsail and EC2. The exact restriction depends on the provider, product, account, region and date, so test from the actual instance rather than assuming that a standard SMTP port is available.
A port block is a network-policy problem, not an error in your mail library. Changing the recipient address, adding a password, or retrying indefinitely will not make a direct connection succeed. You must either submit through a reachable relay or obtain an approved exception from the VPS provider.
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Relay submission versus direct delivery
Direct delivery
With direct delivery, your application or mail transfer agent opens an SMTP connection to each recipient domain’s MX server. The connection normally uses port 25. You are responsible for queueing, retries, reverse DNS, IP reputation, authentication policy, bounce processing, blocklists and domain authentication. If the VPS filters outbound port 25, this architecture cannot work until the provider removes the restriction.
Authenticated relay submission
A relay accepts a message from your VPS after authenticating your account, then delivers it to the recipient network. Your VPS connects to the relay’s submission port, normally 587 with STARTTLS. The relay handles the server-to-server part of delivery. This avoids a port-25 dependency and usually provides delivery logs, bounce events and suppression handling that would otherwise have to be built and operated yourself.
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Email API
An email API is another alternative when a provider offers one. Your application sends a request through the provider’s documented API endpoint, while the provider performs SMTP delivery. Use the API credentials, sender verification process and request format specified by that provider; do not try to use SMTP credentials as API credentials unless the documentation explicitly says they are interchangeable.
Which SMTP port should you use?
| Port | Typical role | TLS mode | Authentication | When to choose it |
|---|---|---|---|---|
| 25 | Server-to-server SMTP delivery | Usually negotiated by the mail servers | Normally not authenticated between independent servers | Only when your provider permits direct delivery and you operate the required mail infrastructure |
| 587 | Authenticated message submission | STARTTLS | Required by most relays | Default choice for a VPS-to-relay connection |
| 2525 | Alternate authenticated submission | Usually STARTTLS when the provider documents it | Required by the relay | Fallback when 587 is blocked or filtered and the relay supports it |
| 465 | Authenticated submission | Implicit TLS from the first byte | Required by the relay | Use only when the provider supports 465 and your client has an implicit-TLS mode |
Port numbers do not determine the security mode by themselves. STARTTLS begins with an unencrypted SMTP connection and upgrades it; implicit TLS starts encryption immediately. Selecting STARTTLS on 465, or implicit TLS on 587, commonly produces a TLS handshake failure.
Set up a relay from your VPS
1. Create and verify a sending identity
Open an account with a relay provider and add the domain or sender address that will appear in the From header. Complete the provider’s verification process before troubleshooting SMTP credentials. A relay may accept a connection but reject a message whose sender identity has not been verified.
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Domain verification is preferable when you send from multiple addresses on the same domain. If you verify only one address, keep the application’s From value exactly aligned with that verified identity unless the provider documents an approved alternative.
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2. Obtain the relay settings
Record the provider’s SMTP hostname, username, password or API token, supported ports and TLS modes. For example, Postmark documents smtp.postmarkapp.com with ports 25, 2525 and 587; it supports STARTTLS or TLS/SSL, and its Server API Token can be used as both the SMTP username and password. Other providers use different hostnames and credential formats, so copy the values from the account’s SMTP documentation.
3. Keep credentials outside application code
Put the hostname, port, credentials and sender address in environment variables or a secrets manager. Restrict the secret’s permissions to the service account that sends mail, and rotate it if it appears in logs, source control or a support ticket.
SMTP_HOST=relay.example.net
SMTP_PORT=587
SMTP_SECURITY=starttls
SMTP_USERNAME=account-or-token
SMTP_PASSWORD=secret-value
[email protected]The names above are application settings, not universal variable names. Your framework may call the TLS setting secure, encryption or require_tls. Confirm that the application actually reads the values at startup and that a stale configuration file is not overriding them.
4. Configure authenticated submission
- Set the relay hostname supplied by your provider.
- Set port 587 and select STARTTLS.
- Enable SMTP authentication and provide the username and password or token.
- Set the envelope sender and visible
Fromaddress to a verified identity. - Set a finite connection and response timeout, then allow your application to retry transient failures without sending duplicates.
If port 587 cannot be reached, change only one variable at a time: first try the provider’s port 2525 with STARTTLS, then port 465 with implicit TLS if it is documented. Do not switch to port 465 while leaving the client in STARTTLS mode.
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5. Publish SPF, DKIM and DMARC records
Add the DNS records specified by the relay for your sending domain. SPF identifies permitted sending services, DKIM lets the provider sign messages, and DMARC tells receiving systems how to evaluate alignment and reporting. These records improve authentication and make failures diagnosable; they do not unblock a network port.
6. Send a controlled test
Send one message to an address you control, inspect the relay’s event or delivery log, and check the recipient’s authentication results. Test both a normal message and your application’s failure path so that a temporary relay outage does not silently discard notifications. Continue monitoring bounces and provider suppression events after the initial test.
Provider options
The following services document SMTP relay options relevant to a blocked-port-25 VPS. Reachable ports and account requirements can change, so verify the provider’s current documentation and test from your instance.
| Provider | Documented ports and TLS | Authentication and API | Verification and operations | Pricing, limits and geography |
|---|---|---|---|---|
| Postmark | Ports 25, 2525 and 587; encrypted SMTP is documented, with STARTTLS or TLS/SSL options. | SMTP hostname smtp.postmarkapp.com; the Server API Token is documented as both username and password. API options are also available. |
Requires a verified sender or domain; use the provider’s delivery and suppression information. | Not stated in the available provider details; verify current pricing, limits and regional availability. |
| Twilio SendGrid | STARTTLS on 587; implicit TLS on 465; port 2525 is documented as a fallback when 25 or 587 is blocked. | SMTP authentication and API options are available; use the credential format assigned to your account. | Verify the sender or domain and use the service’s logs, bounce data and suppression controls. | Not stated in the available provider details; verify current pricing, limits and regional availability. |
| Amazon SES | AWS provides an SMTP hostname and credentials. AWS examples use port 587 to avoid EC2 port-25 timeouts. | Use the SMTP credentials generated for SES; API access is also available through AWS. | Complete the identity-verification and account requirements shown in your AWS region, then monitor bounces and complaints using the service’s controls. | Not stated in the available provider details; verify current pricing, limits and regional availability. |
Choose by the port that your VPS can reach, the TLS modes your mail library supports, the verification workflow your team can maintain, and the quality of logs and bounce controls you need. There is no universal guarantee that 587 is open: a provider can block several SMTP ports, and a security group, firewall or egress policy can add another restriction.
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AWS users who have a legitimate reason to deliver directly can request removal of the default port-25 restriction instead of using a relay. AWS says the request can apply to Lightsail or EC2, but approval is not immediate.
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What to submit
- Sign in as the account root user and open AWS’s email-sending-limitations request process.
- Describe the exact use case, expected traffic and recipients.
- Explain how you will prevent abuse, handle compromised credentials and process bounces.
- Provide the instance and region details requested by AWS.
- Include a static IP and reverse-DNS information if available; these details are optional in the request but important for direct delivery.
AWS says processing may take up to 48 hours. Until the request is processed, configure an authenticated relay so application mail does not depend on an unapproved port-25 path.
Prepare DNS before sending directly
Configure reverse DNS for the sending IP and create a matching forward A record before attempting direct SMTP. The forward and reverse names should identify the same mail host. You will still need correct SPF, DKIM and DMARC policy, queue management, retry behavior and reputation monitoring; opening port 25 alone does not make a self-hosted mail server deliverable.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Test whether outbound SMTP is blocked
Run these checks from the VPS itself, not from a laptop on a different network. Replace the hostname with the relay’s documented SMTP hostname.
Check DNS resolution
dig +short relay.example.netA blank result or a DNS error means the host cannot be resolved. Fix the VPS resolver or hostname before investigating a port.
Check TCP reachability
nc -vz relay.example.net 587
nc -vz relay.example.net 2525
nc -vz relay.example.net 465Run only the ports your relay supports. A successful connection proves that TCP reached the service; it does not prove that TLS negotiation or authentication will succeed. A timeout usually indicates an egress firewall, VPS policy, security group or provider-side filtering. An immediate refusal means the destination is reachable but is not accepting that port or address.
Check the TLS mode
For STARTTLS on 587 or 2525, use:
openssl s_client -starttls smtp -connect relay.example.net:587 -crlfFor implicit TLS on 465, use:
openssl s_client -connect relay.example.net:465 -crlfA certificate exchange and SMTP banner indicate that the basic TLS path is working. Do not treat a successful OpenSSL handshake as proof that your credentials or sender identity are valid; complete a real authenticated send through the application or provider test tool.
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Connection timeout
Confirm the hostname resolves, then test 587, 2525 and 465 individually. Check the VPS firewall, cloud security-group egress rules and provider policy. If all supported ports time out, use the provider’s API or a relay reachable over the network path your application is allowed to use. Do not keep retrying a blocked port, because retries only delay the message and can fill your queue.
TLS handshake failure
Match the client setting to the port: STARTTLS for 587 or 2525, and implicit TLS for 465. A proxy or traffic-inspection device can also interrupt negotiation. Compare the hostname in the TLS connection with the provider’s hostname so certificate validation is performed against the correct name.
Authentication failure
Check the username, password or token for hidden whitespace, expiration and account scope. Some services require a generated SMTP credential rather than the account login. For Postmark, the documented Server API Token is used as both SMTP username and password. Never assume another provider uses the same arrangement.
Sender or identity rejected
Verify the exact address or domain in the From header. A valid SMTP login does not necessarily authorize every sender. Complete domain verification, publish the requested DNS records and remove unverified aliases from the application’s configuration.
Message accepted but not delivered
Inspect the relay’s event log for a bounce, suppression or recipient rejection. Check SPF, DKIM and DMARC alignment and confirm that your application is not sending from an address that differs from the verified identity. Suppressed recipients may require remediation in the provider console before another message can be delivered.
Security and operational practices
- Use authenticated submission and encrypted transport whenever the relay supports it.
- Keep SMTP passwords and API tokens in environment variables or a secrets manager, never in a public repository.
- Limit which application components can send and validate recipient input to prevent abuse of your relay account.
- Log message IDs, response classes and provider event IDs, but redact credentials and message bodies that contain personal data.
- Handle temporary failures with bounded retries and exponential backoff; route permanent bounces to a review process.
- Monitor provider suppression events so repeated sends do not damage deliverability.
- Document the selected port and TLS mode beside the deployment configuration so a future change does not pair 465 with STARTTLS or 587 with implicit TLS.
Which approach should you choose?
Use an authenticated relay when
- Your VPS provider blocks port 25 or several SMTP ports.
- You need dependable application notifications without operating a complete mail server.
- You want provider-managed logs, bounce processing and suppression controls.
- You can verify a sender or domain and publish DNS authentication records.
Use an email API when
- Your application already integrates with HTTP APIs more easily than SMTP libraries.
- The provider’s API offers the events, templates or delivery controls you need.
- SMTP egress is restricted but the documented API endpoint is reachable.
Request AWS port-25 removal when
- You specifically need direct server-to-server delivery.
- You can explain your traffic and abuse-prevention controls to AWS.
- You are prepared to manage reverse DNS, forward DNS, queueing, retries, reputation and recipient feedback yourself.
For most VPS-hosted applications, the practical default is an authenticated relay on port 587 with STARTTLS, followed by 2525 when supported and reachable. Port 465 is a valid alternative only with implicit TLS, while direct port-25 delivery should be treated as a separate, operationally demanding design rather than a quick configuration change.
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