Understand email deliverability
Email deliverability is the ability of your emails to reach recipients’ inboxes. Several factors influence deliverability, including:
- Authentication
- Sending reputation
- Content quality
- Bounce management
- Spam avoidance
A strong deliverability foundation helps ensure your messages are not filtered, rejected, or blocked by mailbox providers.
The primary goal of any email campaign is to drive customer engagement, enhance retention, and improve return on investment (ROI).
This section outlines the most critical elements to help you achieve better results.
Authentication
Proper email authentication verifies that your messages are sent from a legitimate source. Emails without authentication are more likely to be flagged as spam or blocked.
Infobip supports the use of SPF, DKIM, and DMARC to establish trust between your domain and mailbox providers:
- SPF (Sender Policy Framework): Defines which IPs are authorized to send emails on behalf of your domain.
- DKIM (DomainKeys Identified Mail): Digitally signs your emails, confirming they have not been altered during transmission.
- DMARC (Domain-based Message Authentication, Reporting & Conformance): Builds on SPF and DKIM to enforce how your domain handles unauthenticated mail and provides reports on authentication failures.
Enable SPF and DKIM to validate your sender identity and prove that your domain is authorized to send emails. This strengthens your sender reputation and signals to mailbox providers that your emails are legitimate.
To complete your authentication setup, configure a DMARC record. DMARC enforces authentication policies and provides visibility into potential abuse by generating reports on authentication failures and suspicious sending behavior.
Add these records to your DNS configuration and verify them through the Infobip platform.
Spam filters
Mailbox providers use spam filters to protect users from unwanted or malicious emails. Filters analyze sender reputation, subject line wording, link structure, HTML formatting, volume patterns, and recipient engagement.
To avoid being flagged as spam:
- Use a verified sending domain with proper authentication.
- Maintain consistent sending volume and frequency.
- Avoid trigger words or misleading formatting in your subject lines.
- Monitor engagement and remove inactive recipients.
Infobip applies certain filtering safeguards, but message content and targeting remain your responsibility.
Spam traps
Spam traps are email addresses used by Internet Service Providers (ISPs) and anti-spam organizations to identify senders who use outdated or non-permission-based email lists. Their goal is to block potentially harmful or unsolicited email traffic.
Spam traps can be:
- Recycled addresses: From abandoned inboxes.
- Planted addresses: Published online to attract email harvesters.
These traps often end up on bulk email lists that are purchased, scraped, or shared, typically without proper consent.
Senders who use such lists risk hitting a spam trap, which results in immediate flagging and blocklisting by ISPs. Hitting a spam trap damages your sender reputation and impacts deliverability.
How to prevent it:
- Use explicit opt-in methods to collect subscribers.
- Never buy or rent email lists.
- Regularly clean your lists using email validation tools to remove invalid or risky addresses.
Reputation metrics
Mailbox providers evaluate your sender reputation based on how recipients interact with your messages. Positive actions like opens and clicks strengthen reputation, while complaints, bounces, or low engagement damage it.
Infobip helps you monitor your reputation by tracking key metrics:
- Bounce rate (hard and soft)
- Complaint rate
- Open and click rates
- Spam placement indicators
High bounce or complaint rates may trigger throttling or blocking by receiving servers.
Key factors that can negatively impact deliverability:
- Subscriber complaint rate: When recipients mark your email as spam, ISPs treat it as a strong negative signal. Even a small number of complaints can result in throttling or blocking.
- Blocklists: Domains and IP addresses flagged for sending spam. Sending from a blocklisted domain or IP reduces deliverability and can prevent emails from reaching recipients.
- Spam traps: Email addresses used to catch illegitimate senders. These often end up on your list if you use purchased or third-party email lists, or if your opt-in process is weak.
- Bounce rate: A bounce occurs when an email cannot be delivered. A healthy bounce rate should stay below 2 percent. Rates above 5 to 10 percent indicate issues with list quality.
Bounce rates
A bounce rate is the percentage of emails that fail to reach recipients. Bounces are classified as:
- Hard bounces: Permanent delivery failures, typically due to invalid or non-existent email addresses.
- Soft bounces: Temporary delivery failures, often caused by full inboxes or temporary server problems.
To calculate your bounce rate:
Bounce Rate (%) = (Number of bounced emails / Number of sent emails with final statuses) × 100
When an email bounces, the receiving mail server returns an error code, which is used to calculate the bounce rate.
For example:
- 6006 – EC_INVALID_EMAIL_ADDRESS: The recipient's email address is invalid, inactive, or disabled. Check for typos and validate the address before retrying.
For a full list of error codes and their meanings, see the Response status and error codes documentation.
Bounce rate thresholds and consequences
If your bounce rate exceeds 3 percent, you receive a warning notification. If it rises above 5 percent, your registered sender is disabled, and all email sending is paused.
You will be notified with:
- The bounce rate percentage
- The time and date of the block
- Steps to improve deliverability
The affected domain is highlighted in red. After 24 hours, the domain is automatically re-enabled.
High bounce rates signal to mailbox providers that you may be a poor sender. Over time, this can lead to:
- Deliverability issues (such as being marked as spam)
- IP or domain blocklisting
- Damage to both your and Infobip’s sending reputation
How to maintain a healthy bounce rate
To avoid bounce-related disruptions and protect your sender reputation, follow these best practices:
- Only send emails to users who have explicitly subscribed.
- Do not use scraped, purchased, or third-party contact lists.
- Regularly clean and validate your email lists.
- Verify new subscribers' email addresses before sending.
- Respect unsubscribers, do not retarget them.
- Avoid sudden spikes in email volume.
- Ensure your email content is clean and relevant.
Google Postmaster Tools
To monitor your sender domain's performance with Gmail, create a Google Postmaster Tools (opens in a new tab) account and share read-only access with Infobip. This enables alerts when your domains exceed Gmail’s spam complaint thresholds.
You can track:
- Spam rate
- Domain reputation
- Authentication status
- Delivery errors
To access this data, add and verify your sending domain in the Google Postmaster dashboard. Using this tool alongside Infobip analytics provides a more complete picture of your email deliverability health.
Set up Google Postmaster
To set up Google Postmaster:
- Go to Google Postmaster Tools (opens in a new tab) and select Get started.
- Sign in using a Gmail account or your Google Workspace work email.
- Select Get Started again and add your sender domain.
- Google provides a TXT (or optional CNAME) record.
- Add the provided record to your domain’s DNS settings (or ask your IT team to do it).
- Once added, select the three-dot menu next to your domain and select Verify domain.
- After verification, go back to the three-dot menu and choose Manage users.
- Select the red plus button and add the following email as a user:
[email protected]
Learn more about email monitoring, analytics, and performance.