Email Deliverability: The Ultimate Guide to Reaching the Inbox

You send emails, but do they actually reach the inbox? Deliverability is the rate at which your emails land in recipients’ inboxes, as opposed to the spam folder or outright rejection. It’s a critical issue for any business that communicates via email.

Why Your Emails Aren’t Arriving

ISPs (Gmail, Outlook, Yahoo, etc.) use hundreds of signals to decide the fate of each email. Here are the main ones:

1. DNS Authentication

This is the absolute prerequisite. Without it, you’re a stranger to ISPs.

Since February 2024, Gmail and Yahoo require SPF + DKIM + DMARC for bulk senders (>5,000 emails/day). But even for lower volumes, these protocols have become essential.

2. IP and Domain Reputation

ISPs maintain reputation scores for each sending IP and domain. These scores build over time:

• Good reputation: you send regularly, few spam complaints, few bounces
• Bad reputation: irregular sending, high complaint rate, non-existent recipients

Warning: when you start with a new IP, you have zero reputation. You need to build it gradually, this is called warm-up.

3. Blacklists (RBL)

Your IP or domain can be listed on blacklists (also called RBL, Realtime Blackhole Lists). ISPs check these lists to filter spam.

Check if you’re listed with the free Blocklist Checker from Sender Audit.

4. Email Content

Content still plays a role, even though its importance has decreased relative to reputation:

• Deceptive subject lines: artificial RE: or FW:, excessive caps
• Text-to-image ratio: too many images and not enough text = suspicious
• Suspicious links: shortened URLs, domains with no reputation
• Spam words: “free”, “win”, “urgent”, less impactful than before, but still a signal
• Broken HTML: poorly formed HTML code is a negative signal

5. Recipient Engagement

This is the most powerful signal today. Gmail in particular watches:

• Opens: are your emails being opened?
• Clicks: are recipients interacting?
• Replies: the strongest positive signal
• Spam reports: the strongest negative signal
• Delete without reading: a passive negative signal

New Gmail and Yahoo Requirements (2024)

Since February 1, 2024, bulk senders must comply with:

Warm-Up: Building Your Reputation

If you’re sending from a new IP or domain, don’t blast 100,000 emails on day one. Follow a ramp-up plan:

Golden rules of warm-up:

• Start with your best contacts (those who always open)
• Never exceed 2x the previous day’s volume
• Stop immediately if bounce rate exceeds 5%
• Monitor inbox placement (not just send rate)

Continuous Monitoring

Deliverability isn’t a problem you solve once. It’s a metric to watch continuously.

What to Monitor

• Bounce rate: hard bounces > 2% = list hygiene problem
• Complaint rate: > 0.1% = danger, > 0.3% = emergency
• Inbox placement: use seed addresses to test
• Blacklists: regular checks
• DMARC reports: to detect spoofing or alignment issues

With Sender Audit

Sender Audit gives you a complete dashboard to monitor:

• Your authentication score (SPF, DKIM, DMARC, BIMI)
• Your blacklist presence
• Your DMARC and TLS reports
• The overall health of your sending domain

Create your free account and add your domains to the dashboard.

Deliverability Checklist

• SPF configured with -all → check | generate
• DKIM enabled with 2048-bit key → check
• DMARC at quarantine or reject → check | generate
• Not on any blacklist → check
• One-click unsubscribe (List-Unsubscribe)
• Complaint rate < 0.1%
• Reverse DNS (PTR) configured
• TLS enabled on SMTP server
• Mailing list cleaned regularly
• Warm-up followed for new IPs

Questions? Join us on Matrix.