Email marketing deliverability: how to diagnose, clean, and authenticate your list

The critical difference between delivery and deliverability

Many companies confuse two technical concepts that have very different business consequences. Delivery indicates whether the recipient's mail server accepted the message without returning it. Deliverability, specifically inbox placement, measures whether the message reached the main folder and not spam, promotions, or was permanently rejected.

It is possible to have a 98% delivery rate and for a significant portion of contacts to never see the email. According to data cited in the material, in Canada between 15% and 20% of commercial emails do not reach the main inbox. In a 2025 analysis, it was estimated that only 60% of sent emails reached a visible location in the inbox. For a list of 10,000 contacts, this means that up to 2,000 people might not receive the communications.

Diagnosis: Why aren't your emails arriving?

Before changing platforms or redesigning content, it is necessary to identify the root cause. Problems usually fall into three categories:

1. List hygiene: Presence of invalid addresses, hard bounces, and inactive contacts.
2. Technical authentication: Lack of correct configuration of SPF, DKIM, and DMARC.
3. Sender reputation: History of spam complaints, unprocessed unsubscribes, or lack of engagement.

Bounces and list quality

Bounces are classified as:

• Soft bounces: Temporary errors such as a full mailbox or offline server. These can be resolved by retrying the send.
• Hard bounces: Permanent errors such as a non-existent address or invalid domain. Sending to these addresses damages the domain's reputation and should be avoided.

Regular list cleaning is fundamental. Contacts who do not interact (do not open or click) for a prolonged period must be managed through reactivation campaigns or, if they do not respond, removed from the active list.

Technical authentication: SPF, DKIM, and DMARC

Since February 2024, Gmail and Yahoo require that mass senders (sends of more than 5,000 emails daily to personal accounts) authenticate their emails. In May 2025, Microsoft (Outlook, Hotmail, Live) adopted equivalent authentication requirements. Since November 2025, Gmail applies permanent rejections (SMTP 5xx errors) to non-compliant mass emails, eliminating the spam filter as an intermediate option.

These three protocols function as a "digital identity" for email:

Important note: Authentication confirms identity but does not guarantee inbox placement. Providers also evaluate engagement (opens, clicks) and historical reputation.

Consent and unsubscribe management

Consent is not just a legal obligation; it is a deliverability strategy. Sending to people who did not ask to receive emails increases spam complaints and damages reputation.

Key operational requirements

• One-click unsubscribe: Implement the List-Unsubscribe and List-Unsubscribe-Post headers according to the RFC 8058 standard. This allows the user to unsubscribe with a single click, without opening the email.
• Processing timeframe: It is recommended to process unsubscribes within a maximum of 2 business days to align with the deliverability best practices of major providers. In jurisdictions like Canada (CASL), the legal timeframe is 10 business days, but the recommended practice to maintain a good reputation is faster.
• Complaint rate: Keep the spam complaint rate below 0.3%. The recommended operational target is below 0.1%.

Steps to improve deliverability

1. Audit the list: Remove hard bounces and inactive contacts. Use address validation tools before importing new lists.
2. Verify DNS: Check with the hosting or domain provider that SPF, DKIM, and DMARC records are published and valid. Do not modify DNS configuration without consulting the email marketing provider, as they may manage these records for you.
3. Implement one-click unsubscribe: Ensure the RFC 8058 header is present in all emails.
4. Monitor metrics: Track the delivery rate, bounce rate, complaint rate, and unsubscribe rate. A sudden drop may indicate a technical or reputation issue, not necessarily a platform failure.
5. Segment and personalize: Send relevant content to specific segments to increase engagement and reduce complaints.

Common mistakes to avoid

• Attributing the drop in deliverability to the platform: Before switching providers, verify DNS configuration and list quality. Often, the problem is authentication or list hygiene, not the software.
• Ignoring DMARC reports: rua reports provide data on who is sending emails on behalf of your domain. Ignoring them can leave security vulnerabilities undetected.
• Sending to cold lists: Sending emails to contacts who have not interacted in months increases the risk of being marked as spam.
• Confusing delivery with deliverability: A dashboard showing 99% delivery can hide 40% of emails in spam. Use inbox placement tests to measure actual visibility.

Deliverability is a continuous process, not a one-time configuration. It requires constant monitoring, data cleaning, and adaptation to the changing policies of email providers. By combining good technical practices with ethical list management, SMEs can significantly improve the visibility of their communications and protect their sender reputation.