Why are emails delayed when sent from Marketing Cloud on a shared IP despite proper authentication?
Matthew Whittaker
Co-founder & CTO, Suped
Published 5 Aug 2025
Updated 17 Aug 2025
9 min read
I recently encountered a frustrating email deliverability puzzle: why were my emails from Marketing Cloud on a shared IP experiencing significant delays, despite having all my authentication, including DMARC, DKIM, and SPF, properly configured? I was seeing delays of hours, with messages like "Delivered after 22,826 seconds" from Gmail. It felt like I had done everything right on the authentication front, yet my emails were stuck in a digital limbo.
This kind of issue can be incredibly puzzling because authentication is often touted as the be-all and end-all of email deliverability. When emails are delayed despite passing all checks, it points to deeper issues beyond basic setup. This journey into troubleshooting revealed several critical factors that impact email deliverability, especially when using shared IP addresses from an email service provider like Marketing Cloud.
When facing email delays, the first crucial step is to examine the email headers. These headers contain a chronological record of every server that handled your email, along with timestamps. By comparing the Received timestamps, you can pinpoint exactly where the delay occurred, whether it was at the sending ESP, during transit, or at the recipient's inbox provider.
In my case, the headers showed that the delay happened between Marketing Cloud and Google's mail servers. This indicated that either Marketing Cloud was holding onto the email for an extended period before sending it, or Gmail was deferring it repeatedly due to some underlying issue. Understanding this distinction is vital, as it directs your troubleshooting efforts to the correct party.
While checking the headers, I also ran a test that showed an SPF misalignment. However, SPF misalignment often isn't the primary cause of severe delays if DKIM is properly aligned. The key is to check the specific authentication results within the headers, which in my scenario showed both SPF and DKIM passing, as well as DMARC. This suggested that authentication wasn't the root cause of the hours-long delay.
Analyzing email headers
Email headers provide valuable clues to diagnose delivery problems. Look for the Received lines to see timestamps and server information. The difference between the first and last Received lines can indicate where the longest delay occurred. Additionally, check Authentication-Results for DMARC, SPF, and DKIM pass/fail statuses.
Shared IPs and sender reputation
The core of my issue, and a common challenge for many senders, revolved around the use of a shared IP address. While a shared IP can be beneficial for small-volume senders, it comes with inherent risks. Your sender reputation on a shared IP is intrinsically linked to the sending practices of all other users on that same IP pool. If other senders on your shared IP engage in poor practices (e.g., sending spam, high bounce rates, low engagement), it can negatively impact your deliverability, leading to delays or even blocklisting (or blacklisting). Even if your own practices are stellar, you're susceptible to the actions of others.
Email service providers (ESPs) like Marketing Cloud manage these shared IP pools, but their effectiveness in segmenting senders and maintaining high-quality pools varies. ISPs, such as Gmail and Microsoft (as per their new sender requirements), scrutinize IP reputation heavily. If an IP has a questionable history or sudden spikes in volume, emails from it may be subject to greylisting or deferrals. This means the receiving server temporarily rejects the email, asking the sending server to try again later. Repeated deferrals lead to significant delays, like the hours I experienced.
The challenge with shared IPs isn't just about avoiding blacklists (or blocklists), but also managing the subtle impacts on sender reputation that lead to these deferrals. Even if an IP isn't explicitly on a public blacklist, its internal reputation scores with ISPs can be low, causing delays. This is often the case when there's no immediate bounce message, but merely a prolonged delivery time. For new email IPs, there's also an IP warming period where deliverability might be intentionally throttled.
The type of traffic on a shared IP also matters. Marketing Cloud sends a lot of marketing emails, which ISPs typically treat with more scrutiny than transactional emails. If the shared IP pool you're on handles a significant volume of marketing or promotional emails, ISPs are more likely to apply stricter filtering and deferrals. This explains why even with proper authentication, Gmail recipients might experience delays. Marketing Cloud itself even acknowledges that verification emails can be deferred by recipient ESPs.
The role of authentication (and its limits)
Email authentication protocols like SPF, DKIM, and DMARC are fundamental for verifying sender identity and preventing spoofing. They help receiving mail servers trust that an email genuinely came from the domain it claims to be from. I made sure all these were correctly configured for my delegated subdomain, and my headers confirmed they were passing. You can find more information in our guide to DMARC, SPF, and DKIM.
However, passing authentication doesn't guarantee instant inbox placement or prevent delays. Authentication is a baseline requirement, not a silver bullet. ISPs consider a holistic view of sender reputation, which includes:
IP reputation: The sending IP address's history and behavior, especially relevant on shared IP pools.
Domain reputation: The sending domain's history, abuse complaints, and engagement metrics. Tools like Google Postmaster Tools provide insights here.
Content quality: Whether the email content triggers spam filters.
Recipient engagement: How recipients interact with your emails (opens, clicks, replies, or spam complaints).
The true role of DMARC
DMARC helps protect your domain from impersonation by instructing receiving servers on how to handle emails that fail SPF or DKIM alignment. While essential for brand protection and deliverability, a passing DMARC score doesn't override other negative sender signals that might cause delays or deferrals. It ensures your email is recognized as legitimate, but it doesn't guarantee immediate inbox delivery. For more details, explore our guide on demystifying SPF TempError in DMARC reports.
The PTR record revelation and shared IP dynamics
The true culprit in my specific scenario turned out to be a misconfigured PTR (Pointer) record associated with the static IP pool I was provisioned on within Marketing Cloud. A PTR record provides a reverse DNS lookup, mapping an IP address back to a domain name. While not strictly an authentication protocol like SPF or DKIM, a correctly configured PTR record (or reverse DNS) is a strong indicator of a legitimate sender to ISPs.
For my static IP, the PTR record was not properly tied to my sending domain, or it wasn't configured in a way that ISPs would trust, especially for a new or low-volume sender on that specific static IP. ISPs often use PTR records as part of their initial assessment of an incoming email's legitimacy. A missing or mismatched PTR record can lead to increased scrutiny, leading to deferrals and delays, even if all other authentication checks pass. This explains why emails were delayed across all inbox providers, not just Gmail.
The solution, as provided by Marketing Cloud support, was to switch from a static IP pool to a dynamic IP pool. Dynamic IP pools (which are also shared IPs, but with IPs that rotate) typically have established sending histories and reputations. While my individual PTR record might not match my sending domain on a dynamic pool, the overall reputation and history of the pool itself are often sufficient to bypass these initial deferral hurdles. This highlights how complex email deliverability can be, involving many factors beyond just the standard authentication protocols. It's not always about a blocklist or blacklist, sometimes it's about these foundational DNS elements.
Static IP challenges
Reputation building: Requires dedicated IP warming and consistent high-quality sending to establish trust.
PTR configuration: Critical for ISP trust, especially for dedicated IPs. Misconfiguration can lead to delays.
Isolation: Your reputation is solely your own, which means mistakes can have a larger, immediate impact.
Dynamic (shared) IP benefits
Pooled reputation: Benefits from the collective sending reputation of other users on the pool.
Managed by ESP: ESPs actively manage these pools to maintain overall good standing.
Mitigated risk: Individual issues are less likely to cause catastrophic deliverability failures.
The complexity of email deliverability
Email deliverability is a multi-faceted challenge. Even with proper DMARC, DKIM, and SPF authentication, factors like shared IP reputation, recipient ISP policies (e.g., Gmail's deferral mechanisms), and underlying DNS configurations like PTR records can cause significant delays. My experience with Marketing Cloud highlighted that even when the obvious authentication checks pass, deeper technical configurations and the broader context of your sending environment (especially on shared IPs) can profoundly impact when your emails arrive.
This situation underscored the importance of comprehensive monitoring and, sometimes, working closely with your ESP to understand and resolve complex underlying issues. Always keep an eye on your email headers for clues, and don't hesitate to investigate beyond standard authentication checks when facing puzzling deliverability problems.
Views from the trenches
Best practices
Always inspect email headers thoroughly to identify where delays occur.
Monitor your sender reputation regularly, especially on shared IP addresses.
Ensure proper PTR records are configured for your sending IPs or check with your ESP.
Segment your email sends: transactional vs. marketing, to manage reputation more effectively.
Common pitfalls
Assuming full authentication guarantees instant deliverability.
Overlooking shared IP reputation issues, or the 'noisy neighbor' effect.
Neglecting PTR record configuration or not verifying its correct setup.
Not contacting ESP support with detailed header information for deeper diagnostics.
Expert tips
Email Geeks: Gmail defers based on various factors, including IP reputation. If Marketing Cloud doesn't provide deferral reasons, check your IP reputation on relevant sender tools.
Email Geeks: SPF misalignment is common with ESPs, but if DKIM aligns, it often isn't the primary reason for delivery issues.
Email Geeks: Moving from a static, un-warmed IP to a dynamic (shared) pool can improve deliverability because the dynamic pool carries established traffic and history, aiding in reputation maintenance.
Email Geeks: Always confirm PTR records are correctly set up, especially for static IPs, as they are crucial for ISP trust and avoiding delays. If your ESP handles this, ensure they've done it correctly.
Expert view
Expert from Email Geeks says the first step in diagnosing email delays is always to check the email headers to identify the exact point of delay.
2024-09-26 - Email Geeks
Expert view
Expert from Email Geeks says Salesforce Marketing Cloud (SFMC) may hold emails for hours or Gmail might repeatedly defer them, leading to delays. SFMC should provide deferral reasons.
2024-09-26 - Email Geeks
What causes email delays on shared IPs?
Email deliverability is a nuanced field, and delays often stem from a combination of factors beyond just proper authentication. While DMARC, SPF, and DKIM are non-negotiable for establishing trust, shared IP reputation and meticulous DNS configurations, such as PTR records, play an equally vital role. Understanding these intricacies and systematically troubleshooting through email headers is essential for ensuring your messages reach the inbox promptly. As the digital landscape for email marketing evolves, a comprehensive approach to email security and deliverability is more crucial than ever.
Gmail. It felt like I had done everything right on the authentication front, yet my emails were stuck in a digital limbo.
Google's mail servers. This indicated that either Marketing Cloud was holding onto the email for an extended period before sending it, or Gmail was deferring it repeatedly due to some underlying issue. Understanding this distinction is vital, as it directs your troubleshooting efforts to the correct party.
Microsoft (as per their new sender requirements), scrutinize IP reputation heavily. If an IP has a questionable history or sudden spikes in volume, emails from it may be subject to greylisting or deferrals. This means the receiving server temporarily rejects the email, asking the sending server to try again later. Repeated deferrals lead to significant delays, like the hours I experienced.
