Definition:
Deliverability, or email deliverability, is the ability of a sender’s emails to reach recipients’ inboxes rather than being rejected or classified as unwanted mail.
In email marketing, sending a campaign is not enough: receiving servers assess messages and decide whether to accept them and where to place them. Sender reputation, email authentication and sending practices influence that decision.
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The difference between delivery and deliverability
The delivery rate indicates the proportion of sent messages that have not been recorded as bounced. This usually means that the receiving server has accepted the email, but does not confirm that it has placed it in the inbox.
For example, a platform may report that 980 out of 1,000 emails were delivered: a delivery rate of 98%. However, some of those messages could have ended up in the spam folder.
The inbox should also not be confused with the Primary tab. In Gmail, a message categorised under Promotions can be correctly delivered to the inbox; this is not the same as being classified as spam.
What affects email deliverability
Email providers combine different signals when assessing messages. The following factors are worth reviewing:
- Domain and IP reputation: sending history, rejections and complaints can influence how much the sender is trusted.
- Authentication: helps verify that the email uses valid mechanisms to identify its origin and detect potential impersonation.
- Contact quality: invalid addresses generate bounces; sending to people who do not expect the messages can lead to complaints.
- Volume and frequency: sudden increases or excessive frequency can cause problems, particularly without a consistent sending history.
- Content and links: misleading subject lines, suspicious links or messages that do not match what was promised can harm reception.
There is no single word whose removal guarantees avoiding spam filters. The assessment depends on the message as a whole, the sender and the recipient.
The role of SPF, DKIM and DMARC
These mechanisms perform related but different functions:
- SPF: allows a domain to declare which servers are authorised to send email using it.
- DKIM: adds a digital signature that allows the signing domain to be verified and changes to the signed parts of the message to be detected.
- DMARC: checks alignment between the visible sender domain and SPF or DKIM, and allows a policy to be published for messages that fail this check, as well as reports to be received.
Configuring them correctly helps authenticate email, but does not guarantee inbox placement. An authenticated message can still be considered unwanted.
Google’s email sender guidelines explain the requirements applicable to messages sent to personal Gmail accounts.
How to improve deliverability
Improvement starts with sending expected messages to contacts who have requested them and clearly identifying the sender.
Addresses that generate permanent bounces should be removed, unsubscribe requests respected and the unsubscribe process made easy. If part of the list has shown no activity for some time, it is better to review its situation before continuing to send the same campaigns.
It is also important to maintain a consistent frequency and increase volume gradually when starting to use new infrastructure. Changing domains or platforms does not resolve poor sending practices on its own.
How to identify deliverability problems
When investigating a decline, it is worth examining bounces, rejections, spam complaints and authentication results. Breaking down the data by receiving provider helps identify whether the problem affects all recipients or is concentrated in one provider.
Opens and clicks can provide context, but do not prove where a message was placed. Privacy features and automated systems can also affect these measurements.
Tests using seed inboxes and tools such as Google Postmaster Tools provide additional information, although they do not necessarily represent every recipient’s experience.