Category: Troubleshooting deliverability

This guide helps you diagnose and resolve email bounce issues in Sender by identifying bounce types, reviewing your metrics, and taking corrective action.

Symptoms

Your Dashboard shows an elevated Bounce rate, Hard bounce rate, or Soft bounce rate in the Traffic and reach report.

The Total bounces count is increasing after recent sends.

Subscribers you expected to receive your emails have a Bounced status when you filter by Email status on the Subscribers page.

Campaign reports show a high number of bounced recipients, and your Total delivered count is significantly lower than Total emails sent.

Possible Causes

Invalid or nonexistent email addresses — The subscriber list contains addresses with typos, deactivated mailboxes, or domains that no longer exist. These generate hard bounces because the receiving server permanently rejects the message.

Full or temporarily unavailable inboxes — The recipient's mailbox is full, the mail server is temporarily down, or the message exceeds size limits. These produce soft bounces that may resolve on retry.

Domain authentication failures — SPF, DKIM, or DMARC records are missing, misconfigured, or were recently changed. Receiving servers may reject or bounce emails that fail authentication checks.

Recipient domain blocking your sends — A specific mailbox provider (e.g., Gmail, Outlook) is rejecting your emails due to reputation issues, blacklisting, or policy restrictions at the domain level.

Stale or unvalidated subscriber list — A list that has not been cleaned or validated in a long time accumulates invalid addresses, spam traps, and disengaged contacts, all of which increase bounce rates.

Steps to Resolve

Step 1 — Review your bounce metrics on the Dashboard

Go to Dashboard and locate the Traffic and reach report. Check the Hard bounce rate and Soft bounce rate to understand the scope. A high Hard bounce rate indicates permanent delivery failures from invalid addresses. A high Soft bounce rate suggests temporary issues. Note the Total bounces count and compare it against Total emails sent to determine the severity. If bounces are concentrated after a specific send, investigate that campaign first.

Step 2 — Identify bounced subscribers and clean your list

Go to Subscribers and click the Email status dropdown. Select Bounced to filter for all subscribers with a bounced status. Review the list to identify patterns — such as addresses from the same domain or addresses with obvious typos. If you find invalid addresses that were imported without validation, consider running your list through an external email verification service before your next send.

Step 3 — Verify your domain authentication status

Go to Account settings → Domains. Check that your domain shows a green checkmark for all three Authentication indicators: SPF, DKIM, and DMARC. If any checkmark is missing or shows an error, your DNS records need attention. Click Recheck DNS records to refresh the status. Authentication failures can cause receiving servers to bounce your emails, especially if a DNS change was recently made.

Step 4 — Investigate bounces by domain using transactional logs

Go to Transactional emails → Logs. Use the Event type filter and select Bounces to isolate bounce events. Then use the Domain filter to check whether bounces are concentrated on a specific recipient domain. If most bounces come from a single provider, the issue may be a block or reputation problem with that provider rather than a list-wide issue. Cross-reference with an external tool like MXToolbox to check for blacklisting.

Step 5 — Send a test campaign and monitor results

After applying fixes — such as removing invalid addresses, correcting authentication, or resolving a blacklisting issue — send a small test campaign to a segment of known-valid subscribers. Monitor the Dashboard for that send. Check the Bounce rate, Hard bounce rate, and Soft bounce rate. If rates return to normal, the fix is working. Continue monitoring across 2–3 subsequent sends to confirm stability.

How Sender Handles It

Automatic hard bounce suppression — Sender automatically suppresses email addresses that return a hard bounce. These contacts are marked with a Bounced status under Email status and are excluded from all future email sends. You can view them by filtering for Bounced on the Subscribers page.

Soft bounce retry logic — When a soft bounce occurs, Sender retries delivery automatically. If the soft bounce persists after multiple retry attempts, the address may be reclassified and suppressed. You do not need to manually retry soft bounces.

Bounce rate tracking on the Dashboard — Sender calculates and displays your Bounce rate, Hard bounce rate, and Soft bounce rate separately in the Traffic and reach report on the Dashboard. These metrics update with each send, allowing you to detect trends and spikes over time by adjusting the date range filter.

Transactional bounce separation — For transactional emails, Sender tracks Hard bounces and Soft bounces as separate metrics under Transactional emails → Metrics. Bounced transactional contacts are managed independently and appear under the TRANSACTIONAL EMAIL section of the Email status filter on the Subscribers page.

How to Verify the Fix

After applying your fix, send a small test campaign to a segment of engaged, recently active subscribers. Go to the Dashboard and check the Traffic and reach report for that send — confirm that Total delivered is close to Total emails sent and that the Hard bounce rate and Soft bounce rate have returned to acceptable levels. Go to Subscribers, filter by Email status → Bounced, and confirm no new addresses were added after the test send. If bounces persist, revisit your domain authentication under Account settings → Domains and run an external blacklist check.

Common Issues

Bounce rate spikes after importing a new list → The imported list likely contains invalid or outdated addresses. Validate your list using an external email verification tool before importing, and remove any addresses that fail validation.

Bounces concentrated on one recipient domain → The receiving domain may be blocking your sends due to reputation or policy. Use the Domain filter in Transactional emails → Logs to confirm, then check your sending domain against blacklists using MXToolbox.

Authentication checkmarks disappeared after a DNS change → A recent DNS update may have overwritten or removed your SPF, DKIM, or DMARC records. Go to Account settings → Domains and click Recheck DNS records. If any checkmark is missing, re-add the required DNS records.

Emails bouncing despite valid addresses → The recipient's mailbox may be full or temporarily unavailable (soft bounce), or your sending domain may have a reputation issue. Check whether the bounces are hard or soft on the Dashboard, and review the Event type → Bounces log in Transactional emails → Logs for details.

Previously active subscribers now showing Bounced status → Mailboxes can be deactivated over time. Filter by Email status → Bounced on the Subscribers page to review affected contacts. These addresses have been automatically suppressed by Sender and will not receive future sends.

FAQs

Does Sender automatically stop sending to hard-bounced addresses?

Yes. Sender suppresses hard-bounced email addresses automatically. These contacts are marked with a Bounced status and excluded from future sends. You can view bounced subscribers on the Subscribers page by filtering Email status to Bounced.

What is the difference between a hard bounce and a soft bounce?

A hard bounce is a permanent delivery failure — the address does not exist or the domain is invalid. A soft bounce is a temporary failure — the inbox may be full or the server temporarily unavailable. Sender retries soft bounces automatically and suppresses hard bounces.

My emails were delivering fine but suddenly started bouncing. What happened?

A sudden spike in bounces may indicate a DNS change affecting your authentication, a blacklisting event, or a list quality issue. Check your domain authentication status under Account settings → Domains, review your bounce metrics by inspecting the Hard bounce rate and Soft bounce rate on the Dashboard, and run a blacklist check using an external tool like MXToolbox.

How long should I wait before checking if a fix worked?

Send a small test campaign immediately after applying the fix. Monitor the Dashboard metrics for that send. If the Bounce rate has returned to normal levels, the fix is working. Continue monitoring for 2–3 subsequent sends to confirm stability.

Can I reactivate a subscriber who was marked as Bounced?

Subscribers marked as Bounced were suppressed because their address returned a permanent delivery failure. Re-sending to these addresses is not recommended, as it can harm your sender reputation. If you believe the bounce was temporary or the address has been corrected, contact Sender support for guidance.

Where can I see bounce details for transactional emails?

Go to Transactional emails → Metrics to view Hard bounces and Soft bounces totals. For individual bounce events, go to Transactional emails → Logs and select Bounces from the Event type filter. You can further narrow results using the Domain and Campaign filters.

Understanding email clipping helps you ensure your campaigns reach subscribers properly and maintain accurate performance tracking.

Email clipping occurs when Gmail detects that your message exceeds certain size limits. The size limit varies depending on the device your subscribers use to view your emails – desktop devices generally allow up to 102 KB, while mobile devices can be much more restrictive, with some iOS devices limiting emails to just 20 KB.

When your email gets clipped, Gmail also cuts off the tracking pixel that Sender uses to measure open rates, which means your campaign analytics won’t be accurate.

What happens when emails are clipped?

When Gmail clips your email, subscribers see a message like this:

If subscribers click “View entire message,” Gmail opens your email in their web browser, but this version may strip out some styling. Your carefully designed layout, colors, fonts, and formatting may appear broken or simply different from your original design.

Understanding size limits

Gmail’s size limits depend on the device:

• Desktop: Generally around 102 KB
• Mobile: Can range from 20 KB on iOS devices to approximately 75 KB on Android

The 20 KB limit on Gmail’s iOS app is particularly strict and inconsistently applied, making it one of the most challenging environments for email delivery.

Important: The size limit includes all HTML code, text content, CSS styling, and links, but does not include actual image file sizes. Images are hosted on Sender’s servers and loaded separately, so large images won’t directly cause clipping.

Your email might also be clipped if you send multiple test emails with identical subject lines to the same inbox, as Gmail may consolidate these into a conversation thread.

Checking your email size in Sender

Sender automatically monitors your email size and alerts you to potential clipping issues. 

When your email exceeds the recommended size limits, you’ll see a Tips & Suggestions notification in the “Review and schedule” step of your campaign creation process. The warning will show:

Click the “Resolve” button to return to the email campaign editor and make changes if desired.

If your email includes dynamic content or personalization, preview it with different subscriber profiles since the final size may vary.

Size risk levels

• Under 85 KB: Safe from Gmail clipping,
• 85-95 KB: Might be at risk, especially on mobile devices,
• Over 95 KB: Very likely to be clipped – optimization recommended.

How to reduce email size

You can optimize your emails without removing content by making these adjustments:

Simplify your structure

• Consolidate blocks: Combine multiple small text blocks with similar styling into fewer, larger blocks. Four separate text blocks contribute more to code weight than one larger block with the same content.
• Merge sections: If you have multiple column sections in a row, combine them into a single layout rather than stacking separate sections.
• Remove spacing sections: Look for opportunities to eliminate redundant spacing or padding sections.

Clean up content

• Use plain text pasting: When copying from Word documents or websites, paste as plain text (Ctrl+Shift+V or Cmd+Shift+V) to avoid importing unnecessary styling code.
• Remove unused styles: Clean up any redundant CSS rules that accumulated during design.
• Optimize colors and fonts: Using fewer unique colors and fonts reduces overall code weight.

Optimize visual elements

• Remove background images: Section background images add significant code weight. Consider replacing them with solid colors or simple patterns.
• Minimize decorative elements: While images don’t count toward size limits, the HTML code needed to display them does. Focus on images that add value to your message.

Find problem areas

If you’re having trouble identifying what’s causing oversized emails:

1. Duplicate your email template
2. Remove sections or blocks one at a time
3. Check the size after each removal to identify the heaviest elements
4. Focus optimization efforts on components providing the biggest size reductions

Best practices

• Design mobile-first: Since mobile devices have stricter limits, designing for mobile ensures emails work everywhere
• Test regularly: Check email size throughout the design process, not just at the end
• Use Sender templates: Start with our optimized templates designed to stay within size limits
• Monitor performance: Keep an eye on open rates and delivery metrics to catch clipping issues early

Additional causes of clipping

Your message might also be clipped if you send multiple emails to the same inbox with identical subject lines. To avoid this, use unique subject lines when sending test campaigns.

That’s it!

If you got stuck on a specific task or can’t find a way to execute a particular job, contact our support team via LiveChat or [email protected] – we’re here to help 24/7.

Factors affecting email deliverability

Many factors are at play that determine whether or not your emails go into a recipient’s inbox or not.

The key ones include:

• Sender reputation: This is one of the most significant factors impacting deliverability.  Your sending email address, domain and sending IP address should be free from any previous email abuse or spamming history;

• Email content quality: Your email content should be relevant to the recipient and spam-free. E.g., excessive use of capital letters, inclusion of spam words, exclamation marks, and other spammy tactics are a strict no-no that may trigger spam filters;

• List quality: You should only ever be sending to opted-in subscribers who’ve expressed interest in receiving your emails. Cleaning up inactive (dead) subscribers and using best practices like double opt-ins is a must;

• Use of security mechanisms: When you set up secure emailing settings such as SPF and DKIM, it signals that you are a genuine business that is what it claims to be. You can learn more about DKIM/SPF records by clicking here.

• Engagement: Your open, click-through, reply, unsubscribe and report spam rates are all closely monitored by ISPs to determine your audience’s experience. More opens, clicks and replies tell them that they are finding your content valuable, therefore, more chances of your emails getting delivered to inboxes;

• Optimizations: Not optimizing your email for viewing over mobile or with popular email clients can result in lower engagement and poorer engagement metrics;

• Internal firewall issues: Sometimes, especially with corporate and university filters, regular emails look suspicious and are blocked. Check the allow listing information (other providers refer to it as “whitelisting”). If that’s the case, you should alter inbox filtration settings and create an exception/whitelist sendersrv.com.

• Others: spammy subject lines, sudden increase in sending frequency (or volume), excessive use of images and attachments, lack of segmentation, unclear unsubscribe links and more could also impact deliverability negatively.

That’s it!

If you got stuck on a specific task or can’t find a way to execute a particular job, contact our support team via LiveChat or [email protected] – we’re here to help 24/7.