Deliverability problems are one of the most frustrating issues for webmasters and companies running mail services on virtual private servers. When recipients’ inboxes silently drop or filter transactional and marketing mail into spam, revenue, reputation and user trust are at stake. This article walks through practical, technical steps to restore email deliverability on a VPS in Hong Kong or elsewhere, with proven diagnostics and fixes you can apply on a Hong Kong VPS, US VPS or US Server.
Why deliverability breaks — core principles
At the protocol level, reliable delivery is governed by DNS records, SMTP server behavior, IP reputation and content signals. A modern mailbox evaluates mail using a combination of:
- DNS-based identity: PTR/Reverse DNS, SPF, DKIM and DMARC.
- IP and domain reputation: past spam complaints, blocklists (RBLs), shared-IP history.
- SMTP correctness: HELO/EHLO hostname, valid TLS, proper SMTP banners, non-open relay configuration.
- Message content and volume patterns: headers, links, bounce rate, sudden spikes in send volume.
If any of these elements is missing or misconfigured, receiving MTA (Mail Transfer Agents) such as Gmail, Yahoo, Microsoft Exchange and local ISPs may mark messages as spam or reject them outright.
Initial diagnostics — tools and first checks
Before changing configs, collect evidence. Use these tools from your Hong Kong Server or US VPS to identify faults:
- mxtoolbox (online) — checks SMTP, DNS, RBLs.
- mail-tester.com — evaluates SPF/DKIM/DMARC, spam score and content issues.
- dig + short, host, nslookup — verify A, MX, TXT, PTR records from multiple resolvers.
- swaks or telnet/openssl s_client — test SMTP handshake, STARTTLS, banner, HELO.
- Spamhaus, SORBS, Barracuda blocklist lookup — check if your IP is listed.
Record the symptoms: soft-bounces with codes (4xx), hard-bounces (5xx), or silent non-delivery. Bounce messages often contain clues (e.g., “550 5.7.1 SPF fail”).
Immediate red flags to watch for
- Missing DKIM signatures or invalid signatures.
- SPF record absent, too permissive (e.g., v=spf1 +all) or mis-specified.
- PTR (reverse DNS) not set or pointing to different hostname than EHLO/HELO.
- IP listed on public RBLs.
- Port 25 blocked by your cloud provider or ISP.
Step-by-step fixes (technical)
Below are the precise, technical actions you should perform on a Hong Kong VPS, US VPS or US Server to restore delivery. These assume root or equivalent control of the server and DNS zone.
1. Ensure correct reverse DNS (PTR)
Create a PTR record that maps your mail server IP to a fully qualified domain name (FQDN), e.g., mail.example.com, and ensure that FQDN resolves back to the same IP (forward-confirmed reverse DNS). PTR records are set at the IP owner (your VPS provider). Open a support ticket with your provider (e.g., Hong Kong Server support) to set PTR correctly. After change, validate with:
- host IP — should return mail.example.com
- dig +short mail.example.com — should return the same IP
2. Configure SPF accurately
SPF is a DNS TXT policy specifying authorized senders. Keep it strict but complete. Example for a single sending host:
v=spf1 ip4:1.2.3.4 -all
Or if using a provider relay and your VPS:
v=spf1 ip4:1.2.3.4 include:relay.example.com -all
Publish via DNS and test with dig TXT yourdomain.com and SPF validators. Avoid ~all unless you need soft-fail while migrating.
3. Add DKIM signing
Generate DKIM keys and publish the public key as a TXT record under a selector (e.g., default._domainkey.example.com). Configure your MTA (Postfix with opendkim, Exim with dkim-filter, etc.) to sign outbound messages.
- Generate key (2048-bit recommended): rsa-sha256.
- Publish TXT record with the public key and test with online DKIM checkers.
- Verify messages carry a valid DKIM-Signature header after sending a test.
4. Implement DMARC
DMARC connects SPF and DKIM and provides reporting. Start with a monitoring policy:
v=DMARC1; p=none; rua=mailto:dmarc-rua@example.com; ruf=mailto:dmarc-ruf@example.com; pct=100
After reviewing reports and fixing issues, move to p=quarantine or p=reject. DMARC reporting gives visibility into spoofing and alignment problems.
5. SMTP server correctness
Configure Postfix/Exim/Sendmail as follows:
- Set myhostname to the FQDN matching PTR and certificate.
- Enable STARTTLS with a valid certificate (Let’s Encrypt or commercial). Verify TLS with openssl s_client -starttls smtp -crlf -connect ip:25.
- Do not act as an open relay — enforce authentication for submission (port 587) using SASL.
- Configure sane greeting banner and remove generic software versions from banners to prevent fingerprinting.
6. Monitor and remove IP from blocklists
If your IP is listed, follow each list’s delisting process. For example, Spamhaus requires you to identify the cause, fix compromised accounts or open relays, and submit a delist request. Some RBLs provide an automated delist after a waiting period if spam stops. Use RBL queries and maintain logs for appeals.
7. Rate limiting, warm-up and sending patterns
Large sudden volumes from a new or cold IP will trigger reputation filters. If migrating to a Hong Kong VPS or switching from a US Server or US VPS, perform IP warm-up:
- Send small volumes initially—gradually increase over weeks.
- Segment lists by engagement—send to active users first.
- Monitor bounce and complaint rates and throttle accordingly.
8. Feedback loops, BIMI and advanced policies
Subscribe to ISP feedback loops (where available) to receive complaint notifications and promptly remove complainers. For advanced authentication, consider BIMI (requires DMARC enforcement and a verified logo) and MTA-STS to enforce TLS for inbound servers. These improve trust with major providers.
Server and location considerations: Hong Kong Server vs US VPS or US Server
Choosing a region influences latency, local regulations and sometimes deliverability. Key considerations:
- Geographic relevance: Mail from a Hong Kong Server is advantageous for HK or APAC recipients because lower latency and matching regional MX heuristics can help deliverability to local providers.
- Reputation differences: Some RBLs and mailbox providers weigh historical IP reputation; a US VPS with a clean history may perform differently than a newly allocated Hong Kong IP. Always check the IP reputation before deployment.
- Regulatory aspects: Data residency and local anti-spam laws differ; make sure outbound mail practices comply with local regulations on both sides.
- Provider support: A Hong Kong VPS provider that offers PTR management and port 25 unblocking by request simplifies restoration steps.
Operational best practices and monitoring
Ongoing hygiene prevents recurrence:
- Rotate DKIM keys periodically and monitor DMARC reports daily.
- Keep detailed sending logs and subscribe to reputation dashboards (Google Postmaster Tools, Microsoft SNDS).
- Use dedicated IPs for high-volume transactional mail—shared IPs can inherit bad reputation.
- Automate bounce processing and unsubscribe flows to maintain list quality.
When to involve external services
If internal fixes don’t resolve issues, consider:
- Using a dedicated mail relay or reputable ESP for bulk mail while keeping transactional mail on your VPS.
- Hiring deliverability specialists to handle delisting and ISP-specific negotiations.
- Using external monitoring and alerting for RBLs and DMARC reports.
Summary
Restoring email deliverability on a VPS requires methodical verification of DNS identity (PTR/SPF/DKIM/DMARC), correct SMTP behavior, IP reputation remediation and careful sending practices. Whether you operate on a Hong Kong Server, US VPS or US Server, follow the steps above to diagnose and remediate issues: set proper reverse DNS, publish and validate SPF/DKIM/DMARC records, ensure secure and compliant SMTP configuration, warm up new IPs, and monitor RBLs and mailbox provider feedback. These technical measures, combined with ongoing monitoring, will restore and maintain high deliverability for transactional and marketing mail.
For teams evaluating infrastructure, consider the operational benefits of a VPS with easy PTR management and robust network connectivity in your target region. If you’re evaluating options, you can review Hong Kong VPS offerings and DNS/PTR support at https://server.hk/cloud.php and company information at Server.HK.
