Deliverability is the ability to ensure that a very high percentage of the emails you send are not only accepted by receiving servers but also placed directly into the inbox rather than rejected or filtered into spam. Achieving strong deliverability is essential for maximizing the impact of your email program.
Unfortunately, many senders struggle because their emails are either blocked outright or relegated to junk folders. This often happens when they overlook technical requirements, neglect sender reputation, or adopt mailing practices that trigger filtering systems.
To succeed, you must consistently meet both technical standards and reputation benchmarks. That means implementing proper authentication protocols, maintaining a trustworthy sending history, and aligning your practices with recipient expectations.
Here are the key areas to consider so you can meet current sender authentication requirements, protect your reputation, and achieve consistently high delivery rates:
Purchased Lists
The use of purchased mailing lists presents significant compliance and reputational risks. Addresses included may not reflect valid opt-in consent, and in some cases may have been harvested from websites in contravention of data protection principles.
Sending to such lists can undermine domain and IP reputation, elevate spam complaint and bounce rates, and result in blocking or blacklisting by mailbox providers. Organizations are strongly advised to rely solely on organically acquired, permission-based subscription lists to ensure compliance and preserve deliverability.
Adding Recipients
You should only email individuals who have explicitly opted in and expect to receive your messages. Always use a confirmed opt-in (double opt-in) process: send a single confirmation email and require the recipient to click a link to verify their subscription.
This extra step ensures the address is valid, prevents accidental or fraudulent signups, and helps you maintain a high-quality, engaged list that reduces complaints and improves deliverability.
List Quality
Emailing recipients after extended periods of inactivity should be approached with caution. Best practice dictates consistent engagement, as subscribers who have not been contacted within 6–12 months may fail to recall their original consent, thereby increasing the risk of complaints.
Organizations are advised to maintain comprehensive subscription records, including date and method of opt-in, and to implement segmentation strategies that separate inactive recipients. Re-engagement initiatives should be used to validate continued interest, and unresponsive addresses should ultimately be removed to preserve deliverability and compliance.
Bounces
When an email generates a failure notice (bounce), it’s important to determine whether the cause is hard (permanent) or soft (temporary).
Hard bounces (e.g., non-existent addresses, domain does not exist) are permanent failures. These addresses should be removed from your list immediately.
Soft bounces (e.g., full mailbox, temporary server issue, message size too large) may resolve on their own. However, if an address produces repeated soft bounces over multiple campaigns, it should be flagged for review or eventual suppression.
Continuing to send mail to invalid or unresponsive addresses increases cost, damages list quality, and can hurt sender reputation. Mailbox providers track how often you attempt delivery to bad addresses, and persistent issues may lead to throttling, filtering, or outright blocking.
As a guideline, aim to keep your bounce rate below 2%, with 0–1% being ideal for maintaining a strong reputation.
SPAM Complaints
In the event of a spam complaint, no further messages should be sent to the affected recipient. If the complaint appears to be an error, consider reaching out through alternative means to resolve the issue. Ignoring complaints and continuing to send can increase complaint volumes and may lead to blocking or blacklisting.
A complaint ratio of less than 0.1% (1 per 1000 emails sent) should be maintained.
ESP
The use of a dedicated Email Service Provider (ESP) is strongly recommended in preference to transmitting directly from a local internet-connected server. ESPs typically ensure compliance with DNS, IP reputation, and authentication standards, while providing bounce handling, reputation monitoring, and delivery analytics. Such services reduce administrative burden and enhance overall deliverability.
This site remains ad-free through sponsorship by AuthSMTP, whose services are recommended for organizations seeking a compliant, authenticated SMTP solution with strong delivery performance.
rDNS / PTR / HELO
The DNS configuration and HELO / EHLO banner of the sending IP address must be valid and accurate. Invalid or mismatched hostnames may result in message rejection or increased filtering. In addition, ensure that reverse DNS (PTR) records are in place and that SPF, DKIM, and DMARC records are correctly published. These measures are essential for authentication and for maintaining strong deliverability.
SSL / TLS
Sending servers are required to support current TLS protocols. Non-compliance may result in rejection or negative scoring by recipient systems. Proper TLS configuration not only safeguards message integrity and confidentiality but also contributes to maintaining a positive sending reputation and ensuring reliable delivery.
SPF
Sender Policy Framework (SPF) defines the authorized IP addresses, hosts, and services permitted to send email for a given domain. Each sending domain must have an accurate SPF record published in DNS.
A properly configured SPF record is critical for authentication, mitigating spoofing, and maintaining strong delivery reputation.
DKIM
DomainKeys Identified Mail (DKIM) provides cryptographic authentication by attaching a digital signature to each outbound message, verifying that it is authorized for the sending domain.
A properly implemented DKIM record is essential to protect against spoofing, ensure message integrity, and support compliance with modern email authentication standards.
DMARC
Domain-based Message Authentication, Reporting and Conformance (DMARC) is a DNS policy record that leverages SPF and DKIM authentication results. It enables domain owners to define how unauthenticated messages should be treated and provides detailed reporting to enhance visibility, strengthen protection against spoofing, and improve the overall security of legitimate email traffic.
Dedicated IP
Sending your emails from a dedicated, static IP address is generally recommended once you reach volumes of 25,000–50,000 messages per month or more. A dedicated IP lets you build and maintain your own sending reputation, without being affected by other senders who share an IP.
Feedback Data
Enrolment in reputation monitoring services, including Microsoft Smart Network Data Services (SNDS) and Google Postmaster Tools, is recommended to track domain and IP performance. These services provide reporting on reputation, delivery, and authentication issues. Certain ESPs may supply SNDS data directly; however, a minimum send volume—typically 100 or more messages per day to Microsoft recipients—is required before reports are produced.
Send Consistently
Mailbox providers generally favor consistent traffic patterns over irregular, high-volume bursts. Concentrating transmission activity into one or two days per month may trigger anomaly detection systems, which can interpret such spikes as indicators of compromise or unsolicited activity.
To mitigate this risk, organizations should implement scheduling practices that distribute outbound traffic more evenly, gradually ramping up sending volume when initiating new campaigns or IP/domain usage. This phased approach allows reputation systems to adjust and recognize the traffic as legitimate. Once an appropriate baseline has been established, maintaining consistent sending patterns is key to promoting reputation stability and improving the likelihood of sustained inbox placement.
Monitor
Sign up for accounts with major providers like Microsoft and Google so you can test whether your emails are reaching the inbox rather than the spam folder. Regularly sending test messages to these accounts allows you to monitor subject lines, content, authentication results, and formatting from the perspective of a real recipient.
Keeping a few test accounts at different providers also helps you quickly diagnose deliverability issues if they arise, since some problems may affect only specific providers. It’s good practice to check both desktop and mobile apps, as rendering and filtering can vary. In addition, you can use these accounts to subscribe to your own mailing lists and verify that the opt-in, confirmation, and unsubscribe processes work as expected.
Content
Avoid using spammy content in your subject lines, such as excessive punctuation (!!!) or writing in ALL CAPS, as these can trigger filters or discourage recipients from opening your email. Make sure your emails include meaningful text content—do not send messages that consist of a single image, as spam filters cannot interpret the content and recipients may miss important information.
It’s also a good idea to maintain a healthy balance of text and images, use clear and professional language, and avoid overly aggressive marketing phrases (e.g. “Buy Now!” or “Act Immediately!”). Ensuring your content is relevant, authentic, and well-formatted improves deliverability and builds trust with your audience.
Personalise
Rather than sending the exact same email to every recipient, personalize the content where possible. Even small changes, such as addressing the recipient by name, referencing their past interactions, or tailoring offers to their interests, can improve engagement.
Showing content that is more relevant to each recipient not only increases open and click-through rates but also reduces the risk of spam filters flagging identical bulk messages.
Where possible, use segmentation to group recipients by behavior, preferences, or demographics, and send them content that fits their profile. This makes your messages feel more valuable and trustworthy to both recipients and filtering systems.
Include Address / Contact Information
Many email regulations require clear sender identification, often including a valid physical mailing address - for example:
CAN-SPAM Act (USA): Mandates inclusion of a valid physical postal address in every commercial email.
CASL (Canada): Requires a physical address plus additional contact information (such as a phone number, email, or web address) that remains valid for at least 60 days after sending.
GDPR and ePrivacy Directive (EU / UK): While they don’t explicitly require a physical address, they obligate senders to clearly identify themselves and provide contact details sufficient for recipients to exercise their rights (such as withdrawal of consent or data access requests).
Best practice it to always display a registered physical address (street, city, postal code, country). Provide direct contact options (reply-to email, phone number, or support URL).
Place this information in the footer of every message in a readable, accessible format. This transparency signals legitimacy to mailbox providers and recipients alike, reduces spam complaints, and ensures compliance across multiple jurisdictions.
Unsubscribe
Always include a clear and easy-to-use unsubscribe option in the body of your emails. The process should require no more than a single click, without forcing the recipient to log in or provide extra details. Making it simple to opt out not only builds trust but also reduces the chance of recipients marking your messages as spam.
In addition, add a List-Unsubscribe header to your emails. This enables mail clients such as Gmail and Outlook to display a built-in unsubscribe link, which many recipients find more convenient. Ensuring both methods are available helps maintain compliance with regulations (like CAN-SPAM, GDPR, and similar laws) and demonstrates respect for subscriber choice.
It’s also best practice to process unsubscribe requests immediately—or at least within the legally required timeframe—and to provide confirmation that the request was successful. You should never attempt to hide or discourage the unsubscribe process, as doing so damages trust and increases the risk of complaints or blacklisting.
Same Address / Safe Sender
To help recipients recognize your messages, always send from the same domain they originally signed up with. Consistency builds trust and reduces the chance of your emails being mistaken for phishing or unwanted mail. Avoid frequently changing sender email address / domains, as this can confuse subscribers and harm deliverability.
Encourage your recipients to add your sending address to their address book or “safe sender” list. This increases the likelihood of your messages reaching the inbox instead of being filtered.
However, keep in mind that being on a “safe sender” list does not bypass technical checks such as SPF, DKIM, or DMARC authentication. If those validations fail, the message may still be rejected or flagged. You can remind subscribers during signup or in welcome emails, and provide simple instructions for the most common email providers.
Out of Control
It is important to recognize that recipients and their Internet Service Providers (ISPs) may implement proprietary filtering criteria. Certain systems may classify any message not explicitly whitelisted by the recipient as unsolicited, irrespective of the sender’s technical compliance. Consequently, even fully authenticated and properly configured email streams may experience filtering anomalies.
Adherence to best practices - including robust authentication, consistent list management, and responsible sending behavior - remains the most effective strategy to mitigate such risks. While deliverability cannot be absolutely guaranteed, compliance with these standards will significantly reduce the likelihood of delivery failures.
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