Why are my emails failing DMARC and landing in spam during domain warm-up?
Michael Ko
Co-founder & CEO, Suped
Published 24 Jul 2025
Updated 19 Aug 2025
8 min read
Starting a new domain or IP address with an email warm-up strategy is a crucial step to building sender reputation and ensuring your messages reach the inbox. However, it can be incredibly frustrating when, despite your best efforts and what seems like correct DMARC setup, your emails still end up in the spam folder. I've seen this scenario play out countless times, and it often leads to head-scratching moments for even seasoned email marketers.
The common assumption is that once DMARC, SPF, and DKIM are configured, your authentication is solid, and deliverability should follow. While these protocols are foundational for email security, they don't operate in a vacuum. During the sensitive warm-up period, other factors can significantly impact your inbox placement, even if your authentication records appear to be passing.
In this guide, I'll explore the common reasons why emails might fail DMARC or land in spam during domain warm-up, and offer practical steps to diagnose and resolve these issues. From unraveling authentication nuances to tackling list quality, we'll cover what you need to know to get your emails consistently delivered to the inbox.
DMARC (Domain-based Message Authentication, Reporting & Conformance) is the policy layer that builds upon SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail). It tells receiving mail servers what to do with emails that claim to be from your domain but fail SPF or DKIM checks. For your DMARC to pass, at least one of SPF or DKIM must pass and align with your From: header domain. If you want a more in-depth look at these protocols, I recommend reading A simple guide to DMARC, SPF, and DKIM.
A common point of confusion, especially when using an email service provider (ESP), is SPF alignment. Your emails might technically pass SPF checks, meaning the sending server is authorized by the domain in the Return-Path (or Mail From) header. However, if this Return-Path domain is different from your From: header domain, SPF won't align. This is typical with shared IP pools where the ESP manages the SPF record for its own sending domain. You can read more about this in Do we care about SPF alignment?
For DMARC to pass, at least one of SPF or DKIM needs to align. If your SPF isn't aligning due to your ESP's setup, then it becomes critical that your DKIM configuration is flawless and properly aligned. DKIM aligns when the domain in the d= tag of the DKIM signature matches your From: header domain. If both SPF and DKIM fail to align or pass, your DMARC will fail, often leading to emails being rejected or sent straight to the junk folder.
Check your authentication setup
Ensure your SPF, DKIM, and DMARC records are correctly published in your DNS. Use an email testing tool, such as our free About My Email tool, to verify that your emails are passing authentication checks and aligning correctly. Pay close attention to any Return-Path or d= domain discrepancies, especially with third-party ESPs. Even transactional emails can land in spam if these are not correctly set up.
Decoding DMARC reports: identifying the real sources of failure
DMARC aggregate reports are a goldmine of information, detailing all email streams sending from your domain, whether authenticated or not. If you're seeing DMARC failures from sources you don't recognize, like Microsoft Azure or Amazon, this indicates that these platforms are sending emails using your domain. This could be due to:
Hidden services: You might have forgotten about a transactional email provider, an old marketing tool, or an internal system that sends notifications using your domain.
Mail forwarding: When an email is forwarded, authentication headers can break, making the forwarded email appear as unauthenticated from your domain. For instance, Yahoo's mail routing can be tricky with forwarded mail.
Spoofing/Phishing: Malicious actors might be attempting to send emails from your domain. DMARC's reporting helps you identify these threats. If you're on a blacklist (or blocklist), it might be related to spoofing activities.
During warm-up, setting your DMARC policy to p=none (monitor-only) is often recommended. This allows you to collect DMARC reports and identify all legitimate and illegitimate senders using your domain without impacting your deliverability. Only after a thorough analysis of these reports should you consider moving to a stricter policy like p=quarantine or p=reject. If you're wondering how to safely transition your DMARC policy, follow a careful approach.
DMARC reports (RUA and RUF) can sometimes show SPF TempError or DKIM TempError which might lead to DMARC failures. These are often transient issues, but consistent occurrences need investigation. Similarly, warnings from testing tools about images too big are rarely the primary cause of DMARC failures or spam placement, especially if your image sizes are reasonable (e.g., 60KB for a header). Focus on core authentication and reputation factors first.
The critical role of sender reputation and list quality
Even if your DMARC, SPF, and DKIM are perfectly configured and aligned, your emails can still land in the spam folder during warm-up. This is because email deliverability isn't just about technical authentication, but heavily relies on your sender reputation. Domain warm-up is the process of gradually increasing your sending volume to build a positive reputation with mailbox providers. A sudden spike in volume, or sending to an unengaged list, can quickly damage this nascent reputation.
One significant factor impacting sender reputation is how you obtain consent. If your permission comes from a pre-checked box at checkout, you're likely adding recipients who aren't truly interested in your marketing emails. These individuals are more prone to marking your emails as spam, leading to higher complaint rates. High spam complaints, even during warm-up, signal to ISPs that your emails are unwanted, which can severely impact your domain's reputation and lead to high spam complaint rates and poor inbox placement.
Bad practice
Pre-checked opt-in boxes: Leads to collecting uninterested contacts and higher spam complaints.
Sending to old lists: Mailing dormant or unengaged subscribers can trigger spam traps and increase bounces.
Ignoring soft bounces: Can indicate temporary issues that become permanent if not addressed, hurting your standing.
Inconsistent volume: Drastic changes during warm-up can flag your domain as suspicious.
Good practice
Double opt-in or clear consent: Ensures genuine interest, reducing complaints.
Segmenting engaged contacts: Focus on active users first to build positive engagement signals.
Regular list cleaning: Remove inactive subscribers and bounces to maintain a healthy list.
Gradual warm-up schedule: Slowly increase sending volume to establish trust with ISPs.
Another factor is using a shared IP pool. While this can be cost-effective, your deliverability is influenced by the sending habits of others on that same IP. If another sender on your shared IP gets on a major blocklist (or blacklist), like Spamcop, it can affect your emails. While many consumer mailbox providers are sophisticated enough to differentiate between senders on a shared IP, some enterprise filters, like Mimecast, can be less forgiving. This means that a shared IP blocklisting may disproportionately affect your deliverability to specific organizations.
Ultimately, your list hygiene and the quality of your recipient engagement are paramount. Sending to a list with a high percentage of unengaged contacts or invalid email addresses (which can turn into spam traps) will lead to high bounce rates and complaints, regardless of your DMARC setup. This sends negative signals to ISPs and can cause your domain to be flagged, resulting in emails landing in spam. It's often not the technical setup, but the recipient relationship that determines your inbox placement.
Navigating warm-up challenges successfully
Successfully navigating DMARC failures and spam during domain warm-up requires a multi-faceted approach. It's not just about setting up your DNS records, but about truly understanding your email ecosystem and recipient behavior.
Continually monitor your DMARC reports to identify all sending sources. Investigate any unexpected senders and ensure they are properly authenticated. If you cannot get full SPF alignment with your ESP, ensure your DKIM is robust and aligning. Crucially, prioritize building a clean, engaged email list. Review your consent practices to ensure recipients genuinely want your emails. This will reduce spam complaints and improve engagement metrics, which are key signals for mailbox providers.
By focusing on strong authentication, clear sending visibility, and most importantly, nurturing a healthy sender reputation through engaged audiences, you can overcome the challenges of domain warm-up and achieve consistent deliverability.
Views from the trenches
Best practices
Actively analyze DMARC reports (RUA and RUF) to identify all sending sources, both known and unknown, from your domain.
Prioritize list hygiene by focusing on engaged contacts and regularly removing dormant or unengaged subscribers from your sending lists.
Ensure strong DKIM authentication and alignment, as it often compensates when SPF does not align with your 'From:' domain, especially on shared IPs.
Maintain consistent sending volumes during the warm-up period to build trust with ISPs and avoid triggering spam filters.
Document your email sending practices and collect detailed deliverability data from your ESP to present a clear case for any needed strategic changes.
Common pitfalls
Relying on pre-checked opt-in boxes, which can lead to high spam complaints and a damaged sender reputation due to unenthusiastic subscribers.
Over-interpreting technical warnings from testing tools like 'images too big' when the core issue is likely related to sender reputation or list quality.
Ignoring DMARC failures from 'unknown' sources without investigating if they are legitimate internal systems or forwarded mail breaking authentication.
Underestimating the impact of shared IP pools, where other senders' poor practices can negatively affect your deliverability, leading to blocklistings.
Focusing solely on technical configurations (SPF, DKIM, DMARC) while neglecting the critical role of recipient engagement and list cleanliness.
Expert tips
If you're on a shared IP and experiencing blocklistings (or blacklistings) like Spamcop, it's often the fault of the shared pool's reputation, not necessarily your sending practices. Most major mailbox providers are smart enough to differentiate.
For DMARC failures from unexpected sources, consider temporarily setting your DMARC policy to 'p=reject' to force unauthenticated senders to surface themselves, but proceed with caution and a clear rollback plan.
Open rates, even though not the only metric, can still be a loose indicator of inbox delivery. Compare them against other metrics like clicks and revenue, broken down by recipient domain.
Differentiate between SPF 'pass' and SPF 'alignment'. Many ESPs on shared IPs will pass SPF but not align, which is acceptable if DKIM aligns for DMARC.
Sometimes, it's not external spoofing but an overlooked internal mail stream (e.g., password resets, old notification systems) causing DMARC failures, so investigate thoroughly within your organization.
Expert view
Expert from Email Geeks says that the deliverability issues are likely a recipient expectations or permission problem, and that aggregated reports from external tools might not always reflect real data.
July 31, 2024 - Email Geeks
Expert view
Expert from Email Geeks says that focusing on image size is unlikely to be the primary cause of delivery issues; attention should be paid to other factors.
