How to troubleshoot transactional emails going to spam?
Michael Ko
Co-founder & CEO, Suped
Published 31 May 2025
Updated 19 Aug 2025
7 min read
When transactional emails suddenly start landing in the spam folder, it can be a frustrating and confusing experience. These are critical messages, like password resets, order confirmations, or account verifications, that users expect to receive promptly in their inbox. A sudden shift to the spam folder can disrupt user experience, cause support headaches, and even impact your business operations. It feels like something changed, but pinpointing the exact cause can be challenging.
Often, the initial reaction is to wonder if a recent change to your email setup or website is the culprit. While that's a valid starting point, deliverability is a complex ecosystem, and many factors contribute to whether an email reaches the inbox or gets flagged as spam (or ends up on a blocklist).
Troubleshooting this issue requires a systematic approach, diving into various technical and content-related aspects of your email sending. It's about ruling out common problems and identifying any new patterns that might be triggering spam filters. Let's walk through the key areas you need to investigate.
Verify email authentication (SPF, DKIM, DMARC)
One of the most frequent reasons transactional emails end up in spam is improper email authentication. This includes Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting, and Conformance (DMARC). These records tell receiving mail servers that your emails are legitimate and haven't been spoofed. Misconfigurations or recent changes to your DNS provider can easily break these essential setups, leading to immediate deliverability issues.
I always recommend starting here. Verify that your SPF, DKIM, and DMARC records are correctly configured in your DNS. Even a small typo or an outdated record can cause major problems. For example, your SPF record must include all authorized sending IP addresses or services. If you've recently switched email service providers (ESPs) or internal SMTP setups, updating these records is paramount.
Checking your DMARC record
A DMARC record, for instance, typically looks like a TXT record in your DNS. This record specifies policies for handling emails that fail SPF or DKIM authentication.
The p=tag is crucial here. If it's set to reject or quarantine and your authentication fails, your emails will very likely go to spam. Start with p=none to monitor before enforcing stricter policies. You can also review how to safely transition your DMARC policy.
Monitor your sender reputation
Sender reputation is arguably the biggest factor in email deliverability. Mailbox providers assign a reputation score to your sending IP address and domain, influencing whether your emails land in the inbox or spam folder. A sudden drop in inbox placement could indicate a decline in your reputation. This can happen due to an increase in spam complaints, high bounce rates, or sending to unengaged users.
I always encourage people to use tools like Google Postmaster Tools and Microsoft SNDS (Smart Network Data Services) to monitor your domain and IP reputation. These free services provide valuable insights into your spam complaint rates, IP and domain reputation, and DMARC failures. Keeping an eye on these metrics is crucial for proactive troubleshooting. If you see a spike in spam complaints, that's a clear red flag.
Additionally, check if your sending IP or domain has landed on any email blocklists (also known as blacklists). Many mailbox providers use these lists to identify and block mail from senders with poor reputations. You can use a blocklist checker to see if you're listed. If you are, follow the delisting procedures for each blocklist. Understanding what happens when your domain is put on a blocklist is the first step to resolving it.
Even with perfect authentication and a good reputation, your email content can trigger spam filters. Transactional emails are expected to be concise and directly related to a user's action, so they have a higher bar for content relevance. Avoid anything that might make them look like marketing emails, such as promotional language, excessive images, or too many links. Spam filters are always evolving, and what worked yesterday might not work today.
Review your email content for common spam trigger words, unusual formatting, or a high image-to-text ratio. Spammers often use images to bypass text-based filters, so legitimate emails should aim for a healthy balance. Ensure all links in your email are legitimate and point to trusted domains. Even a single suspicious link can harm your deliverability.
Best practices
Clear subject lines: Keep them concise and directly relevant to the transaction.
Minimal content: Focus on essential information, avoiding marketing fluff.
Text-to-image ratio: Favor text over excessive images.
Valid links: Ensure all URLs are safe and functional.
Common pitfalls
Spam trigger words: Words like "free," "guarantee," or excessive exclamation marks.
Broken HTML/CSS: Poorly coded emails can appear suspicious.
Single large image: Emails consisting only of an image with little text.
Irrelevant content: Including promotions in transactional emails.
Recipient engagement plays a significant role. If users consistently delete your emails without opening them, mark them as spam, or move them to trash, mailbox providers interpret this as a negative signal. Conversely, opens, clicks, and replies boost your reputation. Encourage users to add you to their address book or mark your emails as not spam. For more tips on improving content and avoiding spam filters, review this guide from Postmark on reducing spam complaints.
Review your sending infrastructure
The way you send emails is just as important as what you send. If you're using an internal SMTP setup, ensure it's properly configured and maintained. Internal setups can be tricky, as they require careful management of IP reputation, sending volume, and technical compliance. Many organizations opt for a dedicated email service provider (ESP) for transactional emails precisely because these providers specialize in deliverability infrastructure.
One common issue with internal SMTPs is greylisting or throttling, where receiving mail servers temporarily defer delivery, asking the sending server to retry later. While a standard practice, excessive deferrals can indicate that your sending volume or rate is too high for the receiving ISP, or that your IP lacks sufficient trust. Your mail server logs should provide details on deferrals and bounces, offering clues to underlying issues.
If you're using an ESP, check their status pages for any reported outages or deliverability issues. Ensure you're not exceeding any sending limits or rate limits set by the ESP or mailbox providers. Overloading a shared IP, for example, can quickly lead to blocklisting. Even small changes, like a new webform without CAPTCHA, can lead to bots abusing your service and sending spam on your behalf, damaging your IP reputation. Remember to explore ways to avoid emails going to spam generally.
Maintain strong email authentication (SPF, DKIM, DMARC) with regular checks.
Continuously monitor your sender reputation using Postmaster Tools and SNDS.
Keep transactional email content clean, concise, and free of marketing language.
Implement CAPTCHA on webforms to prevent bot abuse and potential spam traps.
Common pitfalls
Failing to update DNS records after switching ESPs or infrastructure changes.
Ignoring spam complaint rates and negative feedback loops.
Including promotional content or excessive images in transactional emails.
Not monitoring mail server logs for deferrals or bounce messages.
Expert tips
If using an internal SMTP, verify all aspects: configuration, DNS, authentication, mail structure, and traffic volume.
Proactively set up SNDS and Google Postmaster Tools to get visibility into IP and domain reputation.
Always check logs for deferrals and bounces; they provide critical diagnostic information.
Switching to a managed infrastructure, like a reputable ESP, can significantly improve deliverability if internal management is lacking.
Expert view
Expert from Email Geeks says to first check if anything has changed recently, such as a new ESP or DNS provider, as these are common initial causes for deliverability issues.
2019-08-12 - Email Geeks
Expert view
Expert from Email Geeks says to ensure any webform tied to the transactional emails is protected by CAPTCHA to prevent bots from flooding inboxes.
2019-08-12 - Email Geeks
The path forward
Troubleshooting transactional emails going to spam can feel like detective work, but by systematically checking authentication, monitoring reputation, refining content, and ensuring your infrastructure is sound, you can pinpoint and resolve most issues. It's an ongoing process that requires attention and adaptation, as mailbox provider algorithms constantly evolve. Stay vigilant, rely on the data from your monitoring tools, and make incremental changes to optimize your deliverability.
Google Postmaster Tools and 
Microsoft SNDS (Smart Network Data Services) to monitor your domain and IP reputation. These free services provide valuable insights into your spam complaint rates, IP and domain reputation, and DMARC failures. Keeping an eye on these metrics is crucial for proactive troubleshooting. If you see a spike in spam complaints, that's a clear red flag.
