Why are transactional emails from a WordPress site using Mailgun being rate limited by Outlook and Yahoo?
Michael Ko
Co-founder & CEO, Suped
Published 6 Aug 2025
Updated 19 Aug 2025
9 min read
Dealing with transactional emails being rate-limited by major mailbox providers like Outlook and Yahoo, especially when sending from a WordPress site through a service like Mailgun, can be frustrating. You set up a reliable email service, expecting smooth delivery for critical messages such as password resets or account confirmations, only to find them delayed or undelivered. This situation is often tied to how these providers evaluate incoming email streams, particularly concerning IP and domain reputation.
The core of the problem lies in rate limiting, a mechanism used by mailbox providers to manage incoming email volume and prevent spam. When an email server receives a message, it quickly assesses its legitimacy based on various factors. If the sending IP or domain raises red flags, the server might temporarily defer the message, leading to delays. For transactional emails, even short delays can severely impact user experience and site functionality.
The error messages typically indicate a problem with IP reputation or unexpected sending volume. For instance, you might see a 4.7.650 error from Outlook.com or a 4.7.0 [TSS04] from Yahoo (Oath). These are soft bounces, meaning the receiving server is asking the sending server to try again later. While the email might eventually get through, the delay undermines the real-time nature of transactional communication.
Understanding rate limiting and shared IPs
Rate limiting is a standard practice for mailbox providers. It helps them efficiently process large volumes of email, filter out unwanted messages, and maintain system stability. Mailgun, as an email service provider (ESP), manages large pools of IP addresses. When you send emails through Mailgun, you're typically using a shared IP, meaning your email traffic is mixed with that of other Mailgun users. This can be both a benefit and a challenge.
For low-volume senders, shared IPs can be advantageous because the collective sending volume from the pool helps to maintain a warm IP reputation. You benefit from the reputation built by other reputable senders using the same IPs. However, if other users on the shared IP engage in poor sending practices, it can negatively impact the IP's reputation, leading to rate limiting for everyone on that IP, including your transactional emails. This is often why even small sites with low volume can experience issues, as they may be unable to build up IP address reputation on their own.
Another factor is the volume and frequency of your sends. Even if your total volume is low (e.g., a dozen emails a day), sudden spikes can trigger rate limiting. Mailbox providers want to see consistent, predictable sending patterns. If a new Mailgun IP is introduced to your sending stream, or if you suddenly send a batch of emails after a period of inactivity, it might be flagged as unusual activity, leading to temporary deferrals from providers like Yahoo or Outlook.
The role of domain and IP reputation
While IP reputation is crucial, your domain reputation plays an equally significant role. If your domain has a history of poor deliverability, moving to an ESP like Mailgun won't instantly resolve all issues. Mailbox providers track domain-level reputation based on factors such as spam complaints, engagement rates, and blocklist (or blacklist) listings. A previously compromised WordPress site, particularly one that was sending directly from its hosting server (e.g., an OVH VPS), may carry a baggage of poor reputation. OVH, for example, is sometimes associated with spam activity due to its policies and user base, which can taint IPs associated with it.
When Mailgun sends emails on your behalf, it uses its own IPs, but the domain in the From header remains yours. This means that if your domain has a bad rap, even a clean Mailgun IP might face scrutiny. Mailbox providers, especially Yahoo, inspect the full content and context of the message, including the domain's history, before deciding on delivery.
The transition from direct PHP mail sending to an ESP like Mailgun is a positive step, but it often requires a reputation warm-up period. Mailbox providers need time to observe the new sending patterns and build trust with your domain. For small-volume sites, this can feel like an endless wait, but patience is key as filters adjust to your new, legitimate email stream. The system needs to learn that your emails are desired by recipients and not spam.
Essential authentication and content factors
Beyond IP and domain reputation, proper email authentication is non-negotiable for transactional emails. This includes SPF, DKIM, and DMARC. When these records are correctly configured and aligned with your sending domain, it signals to mailbox providers that your emails are legitimate and haven't been tampered with. Mailgun provides the necessary DNS records to set these up, and it's essential to verify their correct implementation.
WordPress plugins, while convenient, can sometimes introduce their own challenges. Some plugins might not adhere to email best practices, leading to message creation pathways that trigger spam filters. These 'WordPress specific fingerprints' might appear benign to you but are recognized by filters as patterns often seen in abusive mail. Ensuring your WordPress site is secured against automated attacks, such as subscription bombs where bots sign up with fake email addresses, is vital. These attacks can inadvertently increase your bounce rate and negatively impact your reputation.
Content is also key. While transactional emails are typically brief and to the point, ensure they don't contain elements commonly associated with spam. This includes excessive links, certain keywords, or poor formatting. Regularly reviewing your Mailgun logs for bounces and deferrals, and cross-referencing them with your WordPress activity logs, can help identify unusual sending patterns or issues with specific email content.
Strategies for improvement and recovery
To effectively address rate limiting, several steps can be taken. First, confirm with Mailgun support whether you are on a shared or dedicated IP. For most small WordPress sites, a shared IP is sufficient and often preferred. If your IP has a poor reputation, Mailgun should be able to migrate you to a healthier one, though this is usually handled automatically as part of their service.
Ensure your SPF, DKIM, and DMARC records are correctly set up and aligned with your sending domain. Misconfigurations here are a common cause of deliverability issues. Use a tool to check your email authentication and identify any potential problems. Given the context of using a WordPress site, consider implementing security measures like CAPTCHAs on forms that trigger emails. This can help prevent your site from being exploited by bots, which can lead to unexpected email volumes and complaints.
Patience is a virtue in email deliverability. If you've recently transitioned to Mailgun or if the issue has just started, it's possible that the mailbox providers are simply observing your new sending patterns. Continuously monitoring your Mailgun logs for delivery status and bounce reasons is important. If deferrals persist for more than a week, you might then consider opening a support ticket with Mailgun. For Outlook (Microsoft), you can also use their Postmaster Tools or IP reputation review form, if necessary, though it's best to confirm the IP type with Mailgun first.
Maintaining long-term deliverability
Finally, maintaining good email deliverability is an ongoing process. Even with a powerful ESP like Mailgun, vigilance is required. Regularly monitor your Mailgun dashboard for any warnings or bounce codes. Keep your WordPress plugins updated and ensure your site's security is robust to prevent any unauthorized email triggering. A proactive approach to email deliverability will ensure your critical transactional emails reach your users reliably, without being caught in the clutches of rate limiting or spam folders. Sometimes, the best solution is simply giving the system time to recognize your legitimate traffic and build a consistent positive sending history.
Views from the trenches
Best practices
Always ensure your SPF, DKIM, and DMARC records are correctly configured and aligned for your sending domain, as authentication is foundational for trustworthiness.
Implement security measures like CAPTCHAs or double opt-ins on any WordPress forms that trigger emails, to prevent bots from sending unwanted messages and impacting your reputation.
Monitor your Mailgun logs regularly for bounce and deferral rates; early detection of issues can prevent them from escalating into more severe deliverability problems.
For new sending streams or when using a new ESP, allow a warm-up period for mailbox providers to build trust with your sending patterns and volume.
Regularly review your email content to ensure it aligns with best practices and avoids elements commonly associated with spam, even for transactional messages.
Common pitfalls
Assuming that switching to a transactional email service like Mailgun will instantly fix pre-existing domain reputation issues without a proper warm-up period.
Not distinguishing between shared and dedicated IPs; shared IPs can be affected by other senders' poor practices, leading to unexpected rate limiting.
Overlooking WordPress security vulnerabilities that allow bots to trigger large volumes of illegitimate emails, inadvertently damaging your sending reputation.
Panicking and making drastic changes too quickly when encountering temporary deferrals, instead of allowing time for mailbox providers to adjust to new sending behavior.
Ignoring the role of domain reputation, focusing solely on IP reputation, when both are critical factors influencing email deliverability.
Expert tips
Check for 'WordPress specific fingerprints' in your email headers or content, as these can sometimes trigger spam filters due to past abuse patterns.
For very low volume sending (e.g., a dozen emails/day), a dedicated IP is often overkill and can make it harder to build reputation compared to a well-managed shared IP pool.
Remember that 4xx error codes like 4.7.650 or 4.7.0 are temporary deferrals, meaning emails will eventually be delivered, but delays can still be critical for transactional messages.
If your domain previously had a 'shite' reputation from sending directly via a problematic host, it will take time and consistent good practices to rebuild trust with mailbox providers.
Always contact your ESP's support (like Mailgun) if issues persist, as they can provide insights into their IP pool's performance and suggest specific actions.
Marketer view
Marketer from Email Geeks says: The Mailgun IP showing in the error message is the mail sending IP, and it is likely a shared IP. For very low daily email volume, building IP reputation on a dedicated IP can be difficult, which suggests a shared IP is generally better for this use case.
2024-09-19 - Email Geeks
Expert view
Expert from Email Geeks says: It is important to ensure that the email authentication, such as SPF and DKIM, is correctly aligned with the From domain and that the From domain itself has a good reputation. Also, evaluate the message creation pathway within WordPress plugins, as some may have negative 'fingerprints' that affect deliverability.
2024-09-19 - Email Geeks
Navigating deliverability challenges
Rate limiting of transactional emails from a WordPress site using Mailgun by Outlook and Yahoo is a common challenge, often stemming from a combination of IP reputation, domain reputation, and sending patterns. It is not necessarily an indication of a major problem but rather the mailbox providers' mechanisms at work to ensure inbox quality.
By understanding the nuances of shared IPs, actively managing your domain's reputation, ensuring correct authentication, and being patient during the warm-up phase, you can significantly improve your deliverability. Focusing on these core areas will help you overcome rate limiting issues and ensure your transactional emails reach your recipients promptly and reliably.
Outlook and 
Yahoo, especially when sending from a 
WordPress site through a service like Mailgun, can be frustrating. You set up a reliable email service, expecting smooth delivery for critical messages such as password resets or account confirmations, only to find them delayed or undelivered. This situation is often tied to how these providers evaluate incoming email streams, particularly concerning IP and domain reputation.
Postmaster Tools or IP reputation review form, if necessary, though it's best to confirm the IP type with Mailgun first.
