Email Deliverability
Definition
Email deliverability is the ability of a sent email to successfully reach a recipient’s inbox rather than being filtered into spam, rejected by the mail server, or rejected.
Email deliverability vs. email delivery
These two terms are often used interchangeably, but they describe different things.
Email delivery confirms that a receiving mail server accepted the message. Successful email delivery simply ends at the recipient server accepting the email instead of rejecting it.
Email deliverability describes what happens next. Email deliverability is whether that accepted message lands in the inbox, gets filtered into spam, or is quietly deprioritized.
An email can be “delivered” and still never reach the recipient. For transactional email where a missed password reset or OTP can block a user from accessing their account entirely, that distinction is operationally critical.
Why email deliverability matters
Transactional emails often carry important information that’s time-sensitive. It’s closely tied to a particular action by the user at a specific point in time. The email is required to confirm the transaction or move on to the next step. In this case, poor deliverability directly disrupts the user experience. A message in spam is functionally the same as a message never sent.
Beyond user experience, deliverability affects sender reputation. High complaint rates and bounces compound over time. The more mail that gets filtered, the worse the signals are that get sent back to ISPs, and the harder it becomes to recover inbox placement. Deliverability problems are easier to prevent than to fix after the fact.
Factors that affect email deliverability
Sender reputation
This is the score that receiving mail servers assign to your sending domain and IP based on your sending history. Bounce rates, spam complaint rates, sending volume consistency, and blocklist appearances all play a role in deciding the score.
Email authentication
This is the set of protocols that prove to receiving servers that your email is legitimately sent from your domain. SPF, DKIM, and DMARC are the primary protocols.
SPF (Sender Policy Framework): Specifies which mail servers are authorized to send on behalf of your domain.
DKIM (DomainKeys Identified Mail): Adds a cryptographic signature to outgoing emails that receiving servers use to verify the message hasn’t been tampered with.
DMARC (Domain-based Message Authentication, Reporting, and Conformance): A policy layer built on SPF and DKIM that tells receiving servers what to do when authentication fails, and sends you failure reports.
Bounce rate
This is the percentage of sent emails that fail to deliver. Hard bounces are permanent failures (invalid address, non-existent domain) and soft bounces are temporary (full mailbox, server unavailable). High hard bounce rates damage sender reputation quickly and signal poor list hygiene.
Spam complaints
Spam complaints are recorded when a recipient marks your email as spam. Most ISPs report these back via feedback loops. A complaint rate higher than 0.1% can trigger inbox filtering at major providers.
IP reputation and warm-up
Sending large volumes from a new IP looks suspicious to mail servers. New IPs need a gradual ramp-up in sending volume over days or weeks to establish a positive reputation before high-volume sends.
Blocklists
Blocklists are third-party databases listing IP addresses or domains associated with spam or abusive sending. Landing on a major blocklist can immediately suppress inbox placement. Shared IP environments carry the added risk of being affected by another sender’s behavior.
Email content
Spam filters evaluate message content alongside sender signals. Certain subject line patterns, excessive links, missing plain-text versions, or a poor HTML-to-text ratio can contribute to filtering even when your sender reputation is healthy.
How to test and monitor email deliverability
Deliverability problems are often invisible until they compound. Proactive testing and monitoring catches issues before they affect real recipients.
Check your authentication setup
Use tools like MXToolbox or Google Admin Toolbox to verify that SPF, DKIM, and DMARC records are correctly published and aligned. Misconfigurations here are common and easy to miss.
Run inbox placement tests
Tools like Mail Tester or GlockApps send test emails to seed accounts across major providers and report where messages land. Run these whenever you make changes to your sending setup.
Monitor bounce and complaint rates
Review these metrics per sending domain regularly, not just in aggregate. A spike in hard bounces from a specific domain is often the first sign of a list quality problem or a spoofed sender.
Watch DMARC reports
DMARC aggregate reports show authentication pass/fail rates across all sources sending on your domain. They surface misconfigured third-party senders and early signs of domain spoofing attempts.
Track inbox placement rate over time
Delivery rate alone isn’t a reliable indicator. Inbox placement rate, which is the percentage of delivered messages landing in the inbox rather than spam, is the metric that actually reflects deliverability health. A rate below 90% warrants investigation.