How can I improve transactional email deliverability after a subscription bombing?
Matthew Whittaker
Co-founder & CTO, Suped
Published 1 Jul 2025
Updated 18 Aug 2025
7 min read
Experiencing a subscription bombing can be a jarring event for any email sender, especially when it impacts your transactional emails. These attacks, often fueled by bots or malicious actors, can flood your system with fake sign-ups, leading to an immediate surge in email volume sent to invalid or unengaged addresses. This rapidly inflates bounce rates, triggers spam complaints, and can quickly damage your sender reputation, making it difficult for even critical transactional messages to reach the inbox.
When a transactional subdomain is hit particularly hard, the challenge intensifies because you can't simply pause these essential communications. Password resets, order confirmations, and security alerts must always reach their recipients reliably. Recovering deliverability requires a multi-faceted approach, focusing on immediate damage control, strategic long-term reputation rebuilding, and implementing robust preventative measures.
Addressing the immediate crisis
The first priority after a subscription bombing is to contain the damage and prevent further harm to your transactional email deliverability. This means immediately halting any new unverified sign-ups and identifying the source of the attack.
Swiftly remove all invalid or suspicious email addresses from your sending lists that were generated by the bombing. Sending to these addresses will only perpetuate the problem and further degrade your sender reputation. While you've already taken steps to clean your lists and pause marketing sends, maintaining a pristine list is fundamental for all email types, especially transactional.
Once you've mitigated the active attack and cleaned your lists, proactive communication with major mailbox providers (ISPs) is crucial. Let them know what happened, what steps you've taken, and your plan to prevent recurrence. This transparency can help them understand your situation and potentially ease the path to recovery. For instance, contacting Apple support directly with details of the issue and your mitigation efforts can be particularly helpful if their services are being affected. Provide them with your company name, email domain, affected IP addresses, SMTP errors, and a clear timeline of the incident.
Key immediate actions
Isolate the issue: Stop all new, unverified sign-ups to prevent further list contamination.
Clean your lists: Remove all invalid and suspicious email addresses immediately. This includes any addresses associated with the bombing, even if they appear valid.
Contact ISPs: Reach out to major mailbox providers to explain the situation and demonstrate your proactive response.
Also, be prepared to share logs and evidence of the attack if requested. This helps ISPs verify your account of the incident and can speed up the de-listing process if your IP or domain (or both) ended up on a blocklist (or blacklist). The more information you provide, the better your chances of a quick resolution.
Reinforcing authentication and reputation
Once the initial crisis is managed, the focus shifts to systematically rebuilding your sender reputation. For transactional emails, this means ensuring every legitimate email sent contributes positively to your standing with ISPs. Authenticating your emails is paramount for this.
Ensure your SPF, DKIM, and DMARC records are correctly configured and enforced. These authentication protocols verify that your emails are legitimate and prevent spoofing, which is especially important after an attack that could mimic your sending domain. Misconfigurations or missing records can cause your transactional emails to be flagged as spam, even without a bombing incident. You can find a simple guide to DMARC, SPF, and DKIM for proper setup. If you are experiencing DMARC verification failures from Google and Yahoo, analyzing your DMARC reports can provide valuable insights.
Example SPF recordTXT
v=spf1 include:_spf.example.com -all
Consider sending transactional emails from a dedicated IP address if you don't already. This isolates your transactional email reputation from other sending activities, like marketing emails. If your marketing IP gets blocked, your critical transactional emails will remain unaffected. This separation is key to maintaining stable deliverability for high-priority messages. For more insights on how to improve the deliverability of transactional emails, review general best practices.
Before mitigation
Apple block: Transactional emails to iCloud and me.com users are consistently rejected or sent to spam folders.
High spam complaints: Significant increase in users marking transactional emails as spam, signaling poor list quality.
Increased soft bounces: Temporary delivery failures from various ISPs due to overwhelmed servers or suspicious sending patterns.
Transactional subdomain impact: Even critical transactional emails are not reaching users, causing operational disruptions.
After mitigation
Apple delisting: Improved deliverability to Apple users after direct communication and reputation improvement.
Reduced complaints: Spam complaint rates return to normal levels as legitimate emails are sent to a clean list.
Stable deliverability: Soft bounces decrease, and emails consistently reach inboxes across various ISPs.
Another strategy is to introduce a new transactional subdomain. While your primary transactional subdomain recovers, you can warm up a fresh one and gradually shift traffic to it. This allows critical emails to continue flowing while the affected domain's reputation slowly mends. Remember that recovery often takes time, as ISP algorithms need to see consistent, positive sending behavior over a sustained period.
Proactive defense and monitoring
To prevent future subscription bombings and maintain high transactional email deliverability, implementing robust proactive measures is essential. Your signup process is the first line of defense. Integrate CAPTCHA or reCAPTCHA to deter automated bots, and use a double opt-in process for all new subscriptions.
Double opt-in ensures that only legitimate users who confirm their email address are added to your list, effectively blocking invalid or malicious sign-ups. This greatly reduces the risk of future bombing incidents. For more details on this, see how to prevent subscription bombing.
Beyond signup fortifications, continuous monitoring of your email program is critical. Regularly check your domain and IP addresses against major blacklists and blocklists (such as the various types of email blocklists). Early detection of a listing can allow for quicker remediation before it escalates into a major deliverability crisis. Understand what happens when your domain is on an email blacklist and how to recover. Also, keep a close eye on your spam complaint rates and bounce rates via Google Postmaster Tools and other analytics. Sudden spikes are clear indicators of underlying issues that need immediate attention, otherwise your email will likely fail to reach the inbox.
Ensures all new subscribers are verified, maintaining list hygiene.
Monitor blacklist/blocklist status
Allows for quick action if your domain or IP is listed, minimizing impact.
Track key email metrics
Early detection of deliverability issues through spam complaints and bounce rates.
Putting it all together
Recovering transactional email deliverability after a subscription bombing is a process that demands immediate action, strategic adjustments, and ongoing vigilance. It starts with cleaning your lists and transparent communication with ISPs. This lays the foundation for rebuilding your sender reputation.
Crucially, strengthen your email authentication with SPF, DKIM, and DMARC, and consider using a dedicated IP for transactional sends to segment your reputation. Introducing a new transactional subdomain can provide a temporary workaround while your primary domain recovers. The time it takes to recover domain reputation varies, but consistency is key.
Finally, implement robust preventative measures like CAPTCHA and double opt-in on your signup forms. Continuous monitoring of your blocklist status and email metrics will ensure you catch and address any future issues promptly. By taking these comprehensive steps, you can effectively improve your transactional email deliverability and safeguard your essential communications.
Views from the trenches
Best practices
Act quickly to remove all suspicious email addresses and disable unverified sign-ups after a bombing.
Contact major ISPs directly to explain the situation and demonstrate your mitigation efforts.
Ensure SPF, DKIM, and DMARC are fully implemented and monitored for proper authentication.
Consider a dedicated IP for transactional emails to protect their sending reputation.
Implement CAPTCHA and double opt-in to prevent future automated subscription bombings.
Common pitfalls
Delaying the removal of invalid emails, which further hurts sender reputation and increases spam complaints.
Failing to inform ISPs about the incident, leading to prolonged blocks and slower recovery.
Neglecting email authentication standards, making your legitimate emails look suspicious.
Mixing marketing and transactional email traffic on the same IP, risking both reputations.
Not fortifying signup forms with bot prevention, leaving your system vulnerable to repeat attacks.
Expert tips
If your transactional deliverability is severely affected, consider setting up and warming up a new transactional subdomain as a temporary measure.
Provide detailed logs and evidence of the attack to ISPs to facilitate a quicker de-listing process if your domain or IP is blocked.
Regularly review your email infrastructure for vulnerabilities that could be exploited by bombers.
Maintain a clear separation between marketing and transactional email sending practices and infrastructure.
Educate your team on the importance of email list hygiene and proper acquisition practices to prevent future issues.
Expert view
Expert from Email Geeks says that after cleaning the list and securing signup forms, regaining deliverability mostly takes time. It is recommended to contact Apple Support, providing specific details like company name, email domain, affected IPs, SMTP errors, and a detailed issue description.
June 15, 2021 - Email Geeks
Marketer view
Marketer from Email Geeks suggests exploring the option of setting up a new transactional domain and temporarily routing some traffic there to compare performance.
Google and 
Yahoo, analyzing your DMARC reports can provide valuable insights.
Apple block: Transactional emails to 
iCloud and 
me.com users are consistently rejected or sent to spam folders.
Apple users after direct communication and reputation improvement.
