Email Deliverability is the ability to successfully deliver emails to a recipient's primary inbox without being delayed, bounced, or flagged as spam.

Achieving high deliverability requires a combination of proper email authentication, strict server security, clean domain reputation, and compliant content practices. This guide covers the essential steps to diagnose and fix email deliverability issues.

Inbox providers (such as Gmail, Microsoft 365, Yahoo, and Apple Mail) evaluate three main layers before placing an email in the primary inbox:

# **Authentication Protocols:** Verifying that the sender actually owns the domain and that the message was not altered in transit.
# **Infrastructure & IP Health:** Ensuring the sending mail server has proper reverse DNS, SSL/TLS security, and is clean of blocklists.
# **Domain & Sender Reputation:** Historical engagement data (open rates, spam complaints, bounce rates).

When deliverability drops, inspect non-delivery reports (NDRs) or server logs for 4xx and 5xx errors:

• 550 5.7.1 Access Denied / Rejected: Usually indicates an IP blacklist hit or failed SPF/DKIM/DMARC alignment.
• 550 5.1.1 Unknown User: Indicates high hard bounce rates due to stale or invalid subscriber lists.
• *For a detailed guide on error codes, read SMTP Status Codes & Email Bounces.*

Ensure your domain passes all three core protocols:

• SPF Alignment: Make sure your `MAIL FROM` envelope domain matches your `From:` header domain.
• DKIM Alignment: Verify that your outbound mail client or MTA signs outgoing emails with a valid DKIM selector.
• DMARC Compliance: Ensure at least one protocol (SPF or DKIM) passes and aligns with the `From:` header.

If messages land in the Spam folder instead of bouncing:

• Extract the raw Email Headers from a test message sent to Gmail or Outlook.
• Look at the `Authentication-Results:` line to see if `spf`, `dkim`, or `dmarc` show `fail` or `softfail`.
• Check spam score tags (e.g., `X-Spam-Status` or `SCL` headers).

If your sending IP or domain is listed on major Real-time Blackhole Lists (RBLs) like Spamhaus, Barracuda, or Spamcop:

• Identify and stop the root cause (e.g., compromised user accounts sending spam, open relays, or high spam complaints).
• Request a delisting on the specific RBL provider's portal after fixing the issue.

• Remove Hard Bounces Immediately: Continuously sending mail to non-existent addresses severely damages domain reputation.
• Implement Double Opt-In: Require new users to confirm their email address before sending marketing or operational updates.
• Unsubscribe Mechanisms: Always include a single-click unsubscribe link and a `List-Unsubscribe` header.

You can audit and optimize your email deliverability setup using Telenegar Tools:

• Full Authentication & Mail Scan: Test your SPF, DKIM, DMARC, MX, and SMTP responses using the Telenegar Mail Check Tool.
• DNS Propagation Check: Validate that your authentication TXT records are fully propagated globally via the Telenegar DNS Check Tool.

• Telenegar Mail Check Tool — Diagnostic tool for email authentication and mail server health.
• Telenegar DNS Check Tool — Diagnostic tool for DNS record resolution and propagation.
• SPF | DKIM | DMARC — The core authentication stack.
• SMTP Status Codes & Email Bounces — Common non-delivery codes explained.
• Email Headers — Learn how to parse raw message headers for spam indicators.
• Reverse DNS (PTR) — Set up FCrDNS for outbound mail servers.