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Use an authenticated SMTP relay or an email API instead of connecting directly to recipient mail servers. Configure the relay on port 587 with STARTTLS; use port 2525 if 587 is unreachable, or port 465 with implicit TLS when the provider supports it. Direct delivery requires outbound port 25, which many VPS platforms restrict.
What a blocked port 25 means
SMTP port 25 is the traditional server-to-server route. When your mail server looks up a recipient domain’s MX record and delivers directly, it opens a connection to that recipient’s mail server on TCP 25. A VPS provider may block that connection at the network level, so your application can run normally while every direct delivery attempt times out.
A block can exist in either direction: the hosting company may filter outbound traffic from your instance, or a recipient network may refuse connections from your VPS address. Changing your application’s SMTP setting does not remove a provider-level firewall rule.
Providers restrict port 25 primarily to reduce spam, compromised-account abuse and reputation damage. DigitalOcean’s current policy explicitly states: “SMTP ports 25, 465, and 587 are blocked on Droplets to prevent spam and other abuses on our platform.” AWS blocks outbound port 25 by default on Lightsail and EC2, although AWS allows an account holder to request removal of that restriction.
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Do not assume that 587 is open merely because it is the standard submission port. Policies differ by provider, product, region and account, and some platforms block 25, 465 and 587 together. Test from the actual VPS before choosing a port.
The practical solution: submit mail to a relay
An SMTP relay accepts authenticated messages from your VPS and then delivers them to recipient MX servers. Your server only needs an outbound connection to the relay’s submission endpoint. The relay handles queueing, retries, TLS to destination servers, bounce processing and reputation management.
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| Architecture | VPS connection | What you manage | When to use it |
|---|---|---|---|
| Direct SMTP delivery | Recipient MX servers on port 25 | Mail queue, retries, reputation, DNS, TLS and abuse controls | Only when your provider permits port 25 and you can operate a complete mail server |
| Authenticated SMTP relay | Provider submission port, normally 587 | Application credentials, sender verification and DNS records | Default choice for most VPS applications |
| Email API | HTTPS to the provider | API credentials, request handling, sender verification and event processing | Useful when SMTP ports are filtered or you want structured delivery events |
Which SMTP port should you use?
| Port | Encryption mode | Role | Typical action |
|---|---|---|---|
| 25 | Usually negotiated SMTP encryption where available | Server-to-server delivery | Avoid for application submission unless your VPS provider has explicitly enabled it |
| 587 | STARTTLS (connection begins unencrypted, then upgrades) | Authenticated message submission | Try first when the provider documents it |
| 2525 | Usually STARTTLS; confirm with the relay | Alternate authenticated submission port | Try when 587 is blocked or filtered |
| 465 | Implicit TLS (TLS starts immediately) | Authenticated submission where supported | Use an SSL/TLS client mode, not STARTTLS |
Port and TLS mode must match. Sending STARTTLS commands to an implicit-TLS endpoint produces a handshake error; opening an implicit-TLS connection to a STARTTLS endpoint also fails. A relay may expose several ports but still apply account-level restrictions, so follow its current documentation.
Step-by-step setup with an SMTP relay
1. Create an account and verify your sender
Choose a relay that supports authenticated SMTP or an API. Add the domain you intend to use in the From address, such as example.com, and complete the provider’s verification process. Some services also require verification of an individual sender address.
Do not use an unverified or forged From address. The relay can reject it, and recipients may classify the message as suspicious even when authentication succeeds.
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2. Publish SPF, DKIM and DMARC records
- SPF: Authorizes the relay’s sending infrastructure for your domain. Add the provider’s prescribed mechanism to the domain’s single SPF policy; do not publish multiple independent SPF records.
- DKIM: Lets the provider sign each message. Publish the selector record supplied by the relay, usually as a DNS TXT or CNAME record.
- DMARC: Tells receiving systems how to evaluate SPF and DKIM alignment and what to do when both fail. Start with a monitoring policy if you are still identifying legitimate senders, then tighten it after reviewing reports.
DNS changes can take time to appear at recursive resolvers. The relay dashboard normally shows whether each record is detected; send only after the domain is marked verified.
3. Store credentials outside your source code
Put the relay hostname, port, username and password or token in environment variables or a secrets manager. Restrict file permissions and never commit credentials to a repository or print them in application logs.
SMTP_HOST=smtp.provider.example
SMTP_PORT=587
SMTP_USER=your-smtp-username
SMTP_PASSWORD=your-password-or-token
MAIL_FROM=alerts@example.comUse the exact hostname and credential type supplied by your provider. For example, Postmark documents smtp.postmarkapp.com, ports 25, 2525 and 587, STARTTLS or TLS/SSL, and its Server API Token used as both the SMTP username and password.
4. Configure STARTTLS on port 587
A typical SMTP client sequence is: connect, issue EHLO, request STARTTLS, perform certificate validation, issue EHLO again, authenticate, submit the message and quit. The second EHLO is required by many servers after TLS negotiation.
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from email.message import EmailMessage
msg = EmailMessage()
msg["From"] = os.environ["MAIL_FROM"]
msg["To"] = "recipient@example.net"
msg["Subject"] = "VPS delivery test"
msg.set_content("This is a test message.")
host = os.environ["SMTP_HOST"]
port = int(os.environ.get("SMTP_PORT", "587"))
context = ssl.create_default_context()
with smtplib.SMTP(host, port, timeout=20) as smtp:
smtp.ehlo()
smtp.starttls(context=context)
smtp.ehlo()
smtp.login(os.environ["SMTP_USER"], os.environ["SMTP_PASSWORD"])
smtp.send_message(msg)Keep certificate verification enabled. Disabling it hides hostname or interception problems and exposes credentials to an attacker.
5. Switch to port 2525 when necessary
If TCP 587 cannot be reached and the relay documents 2525, change only the port first and keep STARTTLS enabled. Port 2525 is not a universal standard: the relay must listen there, and your VPS provider must permit it.
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6. Use implicit TLS on port 465 when documented
Port 465 requires an SSL/TLS connection from the first byte. In Python, use an SSL client rather than calling starttls() on a plain SMTP connection:
with smtplib.SMTP_SSL(host, 465, context=context, timeout=20) as smtp:
smtp.login(os.environ["SMTP_USER"], os.environ["SMTP_PASSWORD"])
smtp.send_message(msg)Do not combine SMTP_SSL with starttls(); that attempts two different TLS modes.
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7. Send a controlled test
Send to an address you control at a different mail provider. Check the relay’s activity or event log, the recipient’s headers, and the application log. Verify that SPF and DKIM pass, the visible From address is the one you verified, and that the message did not enter a suppression or bounce list.
Provider options
The following services document the ports or capabilities listed. Ports, account requirements and program availability can change, so confirm the provider’s current settings before deployment.
| Provider | Documented ports and TLS | Authentication | API | Verification and operations | Pricing and limits |
|---|---|---|---|---|---|
| Postmark | 25, 2525 and 587; encrypted SMTP with STARTTLS or TLS/SSL | Server API Token as both username and password | SMTP and API options are documented | Requires a verified sender identity; monitor bounces and suppressions in the provider account | Not stated here; verify current regional pricing, quotas and retention |
| Twilio SendGrid | 587 with STARTTLS; 465 with implicit TLS; 2525 as a fallback when 25 or 587 is blocked | Use the authenticated credentials required by the account | Not specified here; verify the current API and event features | Verify the sender or domain and review delivery, bounce and suppression events | Not stated here; verify current plans, quotas and geography |
| Amazon SES | AWS supplies an SMTP hostname and credentials; AWS examples use 587 to avoid EC2 port-25 timeouts | AWS-generated SMTP credentials | Verify the current SES API options for your region | Verify identities and configure SPF, DKIM and DMARC; monitor bounces and complaints | Not stated here; verify regional limits, sandbox status and pricing |
Choose based on reachable ports, TLS support, authentication method, API requirements, sender and domain verification, event logs, suppression handling, geographic availability and sending limits. A relay generally gives better operational control than running your own Internet-facing mail server, but it does not remove the need for correct DNS, consent-based sending and bounce handling.
Testing whether outbound SMTP is blocked
Run these checks from the VPS, not from your laptop. Replace the hostname with the relay endpoint shown in your account.
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- Resolve the hostname.
getent hosts smtp.provider.example dig +short smtp.provider.exampleIf both commands return no address, fix DNS or use the provider’s exact hostname.
- Test TCP 587.
nc -vz -w 10 smtp.provider.example 587A successful connection proves that the TCP path is open; it does not prove that TLS or authentication will work.
- Test STARTTLS.
openssl s_client -starttls smtp -connect smtp.provider.example:587 -crlfLook for a certificate chain and a successful SMTP greeting. Certificate-name or protocol errors indicate a TLS configuration problem rather than a simple port block.
- Try the documented fallback.
nc -vz -w 10 smtp.provider.example 2525 openssl s_client -starttls smtp -connect smtp.provider.example:2525 -crlfUse 2525 only if the relay supports it.
- Test implicit TLS on 465.
openssl s_client -connect smtp.provider.example:465 -crlfDo not add
-starttls smtpfor this test.
A connection timeout usually means a firewall, routing or provider restriction. An immediate connection refusal means the destination is reachable but no service is listening or an intermediate firewall rejected it. A TLS handshake failure points to the wrong mode, certificate validation or traffic interception. An SMTP authentication error means the network and TLS path worked; check credentials, token scope and sender verification.
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Every port times out
Check the VPS provider’s outbound-mail policy and any local firewall rules such as iptables, nftables or cloud security-group egress rules. Confirm that ordinary HTTPS works and that the relay hostname resolves. If the provider blocks all documented SMTP ports, use the provider’s HTTPS API or move the sender to a service that permits submission.
587 fails but 2525 works
Leave the TLS mode at STARTTLS, change the port to 2525 and record the choice in deployment configuration. Do not silently fall back to unencrypted SMTP.
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465 connects but the client reports a protocol error
You are probably using STARTTLS against an implicit-TLS endpoint. Select an SSL/TLS connection mode from the start, use the provider hostname so certificate validation succeeds, and remove any explicit STARTTLS call.
Authentication is rejected
Check for a copied username, expired password, wrong account region or an API token being used where an SMTP credential is required. For Postmark, the documented Server API Token is used as both username and password. Ensure the account permits the chosen sender and that the domain or address is verified.
The relay accepts mail but recipients do not receive it
Inspect the provider event log for a bounce, suppression or policy rejection. Confirm SPF and DKIM alignment, a sensible DMARC policy, a valid recipient address and a consistent envelope sender. Do not repeatedly retry permanent bounces; remove invalid addresses and investigate complaint or suppression events.
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Use a verified domain, publish DKIM, align SPF and DKIM with the visible From domain, and send only mail that recipients expect. Keep transactional and bulk streams separated when the provider supports separate credentials or streams. Monitor complaint and bounce rates and pause a faulty deployment instead of increasing retries.
When AWS direct delivery is worth requesting
AWS users who have a genuine need to deliver directly can request removal of the port-25 restriction instead of using a relay. AWS instructs the account root user to submit its email-sending-limitations form with a detailed use case, an abuse-prevention plan, instance and region details, and optional static-IP and reverse-DNS information. AWS says processing may take up to 48 hours, so this is not an immediate workaround for a production incident.
Before direct delivery, configure reverse DNS for the sending IP and create a matching forward A record. Without consistent forward and reverse DNS, recipient systems are more likely to reject or distrust the connection. You must also operate queueing, retries, TLS, bounce processing, blocklist monitoring, authentication records and abuse controls yourself.
Request approval only when you can meet those operational obligations. For ordinary application notifications, authenticated submission through a relay is usually faster to deploy and easier to monitor.
Security and reliability checklist
- Use STARTTLS on 587 or 2525, and implicit TLS on 465; never downgrade because a port is convenient.
- Validate the relay certificate and hostname.
- Keep SMTP passwords and API tokens in a secrets manager or protected environment variables.
- Use separate credentials for development, staging and production where possible.
- Restrict which application components can send mail and rate-limit automated jobs.
- Log provider message IDs, response codes and event status, but redact credentials and message content that may contain personal data.
- Handle transient failures with bounded exponential backoff and permanent failures without endless retries.
- Monitor bounces, complaints and suppressions; remove invalid recipients.
- Verify SPF, DKIM and DMARC after every DNS or provider change.
- Retest connectivity after moving regions, changing VPS plans or enabling a new firewall policy.
A quick decision guide
- If your VPS provider allows outbound 587, configure an authenticated relay with STARTTLS.
- If 587 is blocked but the relay documents 2525, use 2525 with STARTTLS.
- If the relay documents 465 and your client supports implicit TLS, use 465.
- If all SMTP submission ports are blocked, use the relay’s HTTPS API.
- If you require direct MX delivery, request port-25 removal where your provider offers that process, then configure reverse DNS and a matching A record before sending.
The durable design is to keep your application independent of a particular port: store the relay host, port and TLS mode in configuration, test them during deployment, and expose provider response codes to your monitoring system. That way a VPS policy change can be handled by changing a verified relay endpoint rather than rebuilding your mail infrastructure.
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