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What could cause email deliverability or domain reputation to suddenly drop?

Matthew Whittaker profile picture
Matthew Whittaker
Co-founder & CTO, Suped
Published 18 May 2025
Updated 13 Oct 2025
7 min read
Experiencing a sudden drop in email deliverability or domain reputation can be incredibly frustrating. It often feels like it happens overnight, without any clear cause. One day your emails are landing in inboxes, and the next they are all going to spam folders. This decline can significantly impact your communication, marketing efforts, and ultimately, your business operations.
The good news is that these drops are usually attributable to specific factors, and once identified, they can often be resolved. Identifying the root cause requires a systematic investigation of your sending practices, email authentication, list quality, and even external influences. Let's delve into the most common reasons why your email deliverability or domain reputation might suddenly tank.
Understanding these potential causes is the first step toward diagnosis and recovery. Often, multiple issues can compound, making it seem more complex than it is. We will explore each factor in detail to provide a comprehensive guide to troubleshooting and restoring your email health.
Effective email deliverability depends on a delicate balance of factors, and even minor changes can have a significant ripple effect on your sender reputation. Being proactive with monitoring and quick to react to any issues is key to maintaining good standing with mailbox providers.

Sudden changes in sending behavior

Any abrupt change in how you send emails can immediately flag your domain or IP address as suspicious to Internet Service Providers (ISPs). These systems are designed to detect anomalies that might indicate spamming behavior.
One common issue is a sudden spike in email volume. If you suddenly send thousands more emails than your usual daily average, ISPs might interpret this as a bulk spam operation, even if your content is legitimate. This is particularly true if you are using a new IP address or ESP, which requires a careful IP warming process. Changing ESPs and using a new dedicated IP without proper warm-up can devastate your reputation.

Importance of IP warming

When using a new IP address, it is crucial to gradually increase your sending volume. Starting with a low volume of highly engaged subscribers and slowly scaling up helps mailbox providers build trust in your new sending source. Skipping this step is a primary cause of immediate deliverability drops.
Even subtle changes to your email content, such as using new keywords, including more images, or adding new tracking links, can trigger spam filters. Content that was previously accepted might suddenly be flagged due to updated algorithms on the ISP's side. Always test new content variations to ensure they do not negatively impact your deliverability.

Poor list hygiene and engagement

The quality of your email list is paramount to maintaining good deliverability and domain reputation. If your list contains a significant number of inactive, invalid, or spam trap addresses, your reputation will suffer.
A sudden spike in bounce rates, particularly hard bounces, is a clear indicator of poor list hygiene. Hard bounces tell ISPs that you are sending to non-existent addresses, which is often associated with spammers. This can lead to a sudden increase in DNS failure and hard bounces. Regularly cleaning your email list to remove invalid addresses is essential.

Healthy list management

  1. Regularly verify email addresses to reduce hard bounces.
  2. Implement a double opt-in process for new subscribers.
  3. Segment and re-engage inactive users, or remove them if unresponsive.

Unhealthy list risks

  1. High bounce rates lead to lower sender scores.
  2. Hitting spam traps can result in immediate blacklisting.
  3. Low open and click rates signal disengagement to ISPs.
google.com logoSpam complaints are perhaps the most damaging factor. Even a small increase can swiftly degrade your domain reputation. Mailbox providers like Google pay close attention to user feedback, and a sudden influx of complaints will push your emails straight to the spam folder. As Microsoft Learn advises, cleaning your email lists and avoiding invalid or inactive addresses is crucial for maintaining a good sender score.
Finally, low engagement rates (opens and clicks) signal to ISPs that your content is not valuable to recipients. This can lead to a gradual decline in deliverability, eventually resulting in emails being routed to spam. If you notice a sudden drop in email open rates, it is a strong indicator that reputation issues might be at play.

Authentication issues and blocklists

Email authentication protocols like SPF, DKIM, and DMARC are critical for proving that your emails are legitimate and not spoofed. A sudden misconfiguration or failure in any of these can severely impact your deliverability. For example, DMARC verification failures often lead to emails being sent to spam or rejected.
Example DMARC record (p=none)DNS
v=DMARC1; p=none; rua=mailto:reports@yourdomain.com; ruf=mailto:forensic@yourdomain.com; fo=1;
If your domain or IP address is listed on an email blocklist (also known as a blacklist), it will dramatically impact your deliverability. These lists are maintained by various organizations to track and block known sources of spam. A sudden drop in deliverability is a strong indicator you might be on one. We offer blocklist monitoring to help you stay informed.
There are many types of email blocklists, ranging from public to private, and real-time blocklists (RBLs) to DNS-based blocklists (DNSBLs). Getting listed on one means many mail servers will automatically reject your emails. It is essential to use a blocklist checker regularly to proactively identify and address any listings. If your IP gets blocklisted, immediate action is required.

External factors and account compromises

Sometimes, the cause of a deliverability drop isn't directly related to your sending practices but rather to factors outside your immediate control or unexpected events.
If you are using a shared IP address, your reputation is tied to the sending behavior of other users on that IP. If another sender on your shared IP starts sending spam or experiences a major issue, it can negatively affect your deliverability, even if your practices are pristine. The poor IP reputation management by your mail provider could be the cause.

Issue

Impact

Example

Shared IP issues
Reputation dip from other users on the same IP.
yahoo.com logoEmails blocked by Yahoo due to a shared IP blacklisting.
Account compromise
Your server sending phishing emails without your knowledge.
ISP policy changes
New filtering rules affecting previously good emails.
gmail.com logoGmail's new requirements for bulk senders.
A compromised email account or server can lead to a sudden and drastic drop in reputation. If spammers gain access to your sending infrastructure, they can use it to send out large volumes of unsolicited emails, quickly tanking your domain's reputation. This highlights the importance of strong security practices, including secure passwords, two-factor authentication, and regular security audits.
microsoft.com logoMailbox providers frequently update their spam filtering algorithms and policies. A policy change from a major provider like Microsoft or Google could inadvertently affect your legitimate emails, especially if your sending practices are borderline. Monitoring announcements from these providers and adjusting your strategies accordingly is a good proactive measure.

Views from the trenches

Best practices
Monitor your DMARC reports daily to quickly identify any authentication failures or abnormal sending patterns.
Segment your audience and send relevant content to engaged subscribers to maintain high engagement rates.
Regularly clean your email lists to remove inactive users and hard bounces, preventing reputation damage.
Ensure all email authentication records (SPF, DKIM, DMARC) are correctly configured and aligned.
Warm up new IP addresses or sending domains gradually to build a positive sending history with ISPs.
Common pitfalls
Ignoring sudden increases in bounce rates, which can quickly degrade sender reputation.
Sending to unengaged segments or purchased lists, leading to high spam complaints and low opens.
Neglecting DMARC monitoring, missing critical authentication failures and potential spoofing attempts.
Failing to adapt to new ISP requirements, such as Google and Yahoo's 2024 sender policies.
Assuming an AWS IP blacklist is irrelevant, while increased spam reports are the real underlying issue.
Expert tips
If your deliverability suddenly tanks, review recent changes in sending volume, content, or list acquisition immediately.
High spam complaints often indicate a problem with audience targeting or content relevance, not just authentication.
A sudden shift from green to yellow in Google Postmaster Tools for reputable senders often precedes more severe drops.
Investigate any reports of compromised accounts or unauthorized sending from your domain as a top priority.
Understand that blocklist listings, even on less influential lists like UCEProtect, can sometimes correlate with other underlying issues like increased spam complaints.
Marketer view
Marketer from Email Geeks says they observed their domain and IP reputation drop to red in Google Postmaster Tools around January 15th, 2020, despite no significant changes in email production.
2020-01-20 - Email Geeks
Marketer view
Marketer from Email Geeks says they saw a dip in domain reputation to low since late December, with no major changes in their mailing behavior.
2020-01-21 - Email Geeks

Proactive monitoring and swift action

A sudden drop in email deliverability or domain reputation can be alarming, but it is often a solvable problem once the cause is identified. By diligently monitoring your sending patterns, maintaining a clean and engaged email list, ensuring proper email authentication, and being aware of external factors, you can mitigate many potential issues.
suped.com logoThe key is to be proactive rather than reactive. Tools like Suped provide robust DMARC monitoring and blocklist monitoring to give you visibility into your email ecosystem, helping you spot issues early and take corrective action before they escalate.
Regularly reviewing your email performance metrics, understanding Google Postmaster Tools Domain Reputation, and maintaining a comprehensive approach to email health are the best defenses against sudden drops. With the right strategies and tools, you can ensure your messages consistently reach their intended recipients.

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What you'll get with Suped

Real-time DMARC report monitoring and analysis
Automated alerts for authentication failures
Clear recommendations to improve email deliverability
Protection against phishing and domain spoofing