Email Service Providers (ESPs) play a critical role in ensuring their clients' email deliverability. This hinges significantly on the correct implementation and maintenance of foundational technical elements like HELO, rDNS, and SPF. These protocols work in concert to verify the sender's identity, preventing spoofing and improving inbox placement. Best practices involve meticulous configuration and ongoing monitoring to ensure these records accurately reflect sending infrastructure and adhere to evolving internet service provider (ISP) requirements.
Email marketers often grapple with the technical intricacies of email authentication, relying heavily on their ESPs to manage HELO, rDNS, and SPF. Their primary concern is often deliverability and maintaining a strong sender reputation, without necessarily diving into the granular technical details. They look for ESPs that handle these complexities seamlessly, especially when dealing with dedicated IPs and subdomains.
Marketer view
Email marketer from Email Geeks notes the initial confusion when reading the SPF specification, particularly the section discussing HELO checks appearing before MAIL FROM checks. This order can seem counter-intuitive to those accustomed to prioritizing the sender address.
Marketer view
Email marketer from HighLevel Support Portal explains that their guide helps users avoid the spam folder by landing emails in the inbox. This indicates a strong reliance on proper technical setup, including SPF and other authentication methods.
Email deliverability experts emphasize that while SPF may have historical quirks, its adherence to SMTP protocol order and its role in basic authentication remain crucial. They highlight the importance of rDNS (and PTR records) as a fundamental aspect of good engineering practice, especially for ESPs. Proper configuration of these elements is not just about avoiding blacklists (or blocklists) but also about providing clear accountability for email traffic. For more on how email authentication impacts deliverability, see our guide on DMARC, SPF, and DKIM.
Expert view
Expert from Email Geeks explains that the order of HELO and MAIL FROM checks in SPF is intentional because it reflects the sequential nature of the SMTP conversation. The HELO command is issued first by the sending server, establishing its identity before the email transaction proceeds.
Expert view
Expert from SpamResource highlights that the foundation of email authentication relies on correctly identifying the sending server. This means that a properly configured HELO or EHLO value, which matches the rDNS of the sending IP, is a critical trust signal for receiving mail servers. Without this basic alignment, even SPF checks may be viewed with suspicion.
Official documentation and industry standards provide the foundational rules for HELO, rDNS, and SPF. These documents outline the technical specifications that ESPs must adhere to for proper email authentication and reliable delivery. Key findings often revolve around the order of operations in SMTP and the strict limitations on DNS lookups for SPF validation. Understanding these guidelines is paramount for any email infrastructure, whether managed directly or through an ESP. We provide further insights on these topics in our detailed articles, such as how to set up DMARC, DKIM, and SPF.
Technical article
Documentation from Mailgun states that SPF implementation must limit the number of mechanisms and modifiers that do DNS lookups to at most 10 per SPF check. This is a critical technical constraint that ESPs and domain owners must adhere to in order to avoid authentication failures and ensure emails are delivered reliably.
Technical article
Documentation from Mutant Mail emphasizes the importance of regularly reviewing and updating all records related to email delivery, including MX records, SPF, and DKIM, as well as understanding the significance of rDNS and PTR records. This holistic approach ensures comprehensive email deliverability.
12 resources
What is the best practice for reverse DNS resolution when sending email via an ESP?
How to set up DMARC, DKIM, and SPF for emails from a web server and manage bounce responses?
A simple guide to DMARC, SPF, and DKIM
How do SPF, DKIM, DMARC, and dedicated IPs affect email deliverability when using a third-party ESP?
How to troubleshoot SPF authentication issues with multiple ESPs, subdomains, and CNAMEs?
Why your emails fail at Microsoft: the hidden SPF DNS timeout
Boost Email Deliverability Rates: Technical Solutions from Top Performing Senders
Demystifying the SPF TempError in your DMARC reports
Why Your Emails Fail: Expert Guide to Improve Email Deliverability [2025]
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