Skip to main content

Email Deliverability Improvement Guide

I. Core Understanding of Deliverability​

Email deliverability is not the same as "whether an email was sent successfully."

From the perspective of mailbox providers, what truly matters is:

Whether they trust the sender and are willing to place the email in the inbox

Mainstream providers (such as Gmail, Outlook, and Yahoo) dynamically evaluate sender trustworthiness based on historical behavior, then determine final placement (Inbox / Spam / Rejected).

Therefore, deliverability is essentially a "trust management" issue, not just a technical issue.

II. Core Factors Affecting Deliverability​

1. Sender Reputation​

Sender reputation is the foundation of deliverability and consists of domain reputation and IP reputation.

Domain Reputation​

Recommendations:

  • Use a dedicated sending subdomain (for example, mail.yourdomain.com)
  • Separate transactional and marketing traffic by different domains
  • Keep domains stable for long-term use and avoid frequent changes

Domain is the core carrier of long-term reputation. Once damaged, recovery usually takes a long cycle.

IP Reputation​

Recommendations:

  • Use IP addresses with a clean history
  • Avoid polluted shared IP pools
  • Control sending pace and avoid sudden traffic spikes

IP reputation is more like "short-term behavior reputation" and is sensitive to sending pace changes.

2. Email Authentication Mechanisms (Essential Foundation)​

Complete email authentication is a prerequisite for trust. It is recommended to configure at least:

  • SPF: Declares authorized sending servers
  • DKIM: Signs email content to ensure integrity
  • DMARC: Defines authentication policy and reporting

Basic setup principle:

SPF + DKIM + DMARC = Standard Essential Combination

Missing or incorrect setup will significantly reduce delivery success rates.

3. Warm-up​

Warm-up is required in the following scenarios:

  • Newly enabled domain or IP
  • Re-enabling after a long period of inactivity

Recommended strategy:

  • Start with a small sending volume (for example, 50-100 emails/day)
  • Increase daily volume gradually (20%-50%)
  • Prioritize sending to active users
  • Keep sending stable and continuous

The core goal of warm-up is to let mailbox providers build trust gradually, rather than triggering risk-control mechanisms.

4. List Quality​

List quality directly determines feedback quality and is one of the key factors affecting deliverability.

Recommended practices:

  • Send only to users with explicit subscription consent
  • Regularly clean invalid and bounced addresses
  • Filter long-term non-engaged users

Behaviors to avoid:

  • Buying or renting email lists
  • Scraping unauthorized data
  • Leaving send lists unmaintained for long periods

High-quality lists usually mean lower complaint rates and higher engagement.

5. Content and Sending Strategy​

Content Standards​

  • Avoid obvious spam characteristics (exaggerated promotional terms, manipulative phrasing)
  • Keep HTML structure standardized and compatible with mainstream email clients
  • Control image-to-text ratio
  • Provide a clear and usable unsubscribe entry

Sending Strategy​

  • Control sending frequency and avoid burst-style bulk sends
  • Send in batches by provider (for example, separate Gmail/Outlook control)
  • Keep sending rhythm stable and avoid major fluctuations

Content and strategy jointly affect user feedback behavior, which indirectly affects deliverability.

III. Key Metric Reference Ranges​

In daily operations, overall health can be assessed with the following metrics:

MetricRecommended Range
Delivery Rate>= 90%
Inbox Rate>= 70%
Open Rate>= 10% (marketing scenarios)
Complaint Rate< 0.1%
Bounce Rate< 2%

When any metric stays abnormal, prioritize troubleshooting the corresponding area.

IV. Advanced Optimization Strategies​

1. Domain Isolation​

It is recommended to use separate subdomains for different email types, for example:

  • Transactional emails: mail.yourdomain.com
  • Marketing emails: marketing.yourdomain.com

This prevents negative feedback from marketing traffic from impacting critical notification traffic.

2. Domain Alignment​

Ensure sending and tracking domains stay aligned, for example:

  • Sending domain: mail.yourdomain.com
  • Link tracking domain: track.yourdomain.com

A good alignment strategy helps pass DMARC checks and improve trust.

3. Provider-Specific Optimization​

Different providers focus on different signals:

  • Gmail: Focuses more on user engagement (opens, clicks)
  • Outlook: More sensitive to complaints
  • Yahoo: Pays more attention to sending frequency and stability

Sending strategy can be fine-tuned for each provider.

4. Behavior-Based Sending Strategy​

It is recommended to prioritize highly active users:

  • Users with recent opens
  • Users with recent clicks

At the same time, gradually reduce sends to low-activity or non-engaged users.

This approach can significantly improve overall engagement and sender reputation.

V. Common Issues and Troubleshooting Directions​

When deliverability declines, troubleshooting usually starts from the following areas:

1. Technical Configuration Issues​

  • SPF / DKIM / DMARC missing or misconfigured
  • DNS resolution anomalies

2. Abnormal Sending Behavior​

  • Sending volume spikes in a short time
  • Unstable sending rhythm

3. List Quality Issues​

  • Complaint rate increases
  • Too many invalid addresses causing bounce rate increase

4. Content Quality Issues​

  • Copy or design is recognized as promotional spam-like content
  • Abnormal links or links to low-reputation domains