Summary
Email deliverability can be a complex challenge, especially when different email providers enforce compliance rules with varying strictness. A common scenario arises when emails send successfully to Outlook recipients but bounce from Gmail with RFC5322 errors. This usually indicates a structural issue within the email headers that, while tolerated by some Mail Transfer Agents (MTAs) like Outlook, is flagged as non-compliant by stricter ones like Gmail.
Key findings
- Gmail's strictness: Gmail often adheres more rigidly to email standards like RFC5322, leading to rejections for malformed headers that other providers might accept.
- Header non-compliance: The primary cause of RFC5322 errors is often duplicated email headers which violate the standard, or other structural issues within the message.
- Outlook's leniency: Outlook's email servers tend to be more forgiving of minor RFC5322 non-compliance, allowing emails to pass through despite potential issues.
- Self-correction: The issue almost always lies with the sender's email formatting or sending system, not with Gmail's policies.
Key considerations
- Inspect headers thoroughly: Carefully examine the email headers for any duplicated fields or formatting errors. Tools for analyzing email headers can be very useful here, and can help to troubleshoot email delivery issues related to RFC compliance errors.
- Validate compliance: Ensure your sending system is generating emails that are fully compliant with RFC5322. Pay particular attention to unique header requirements, such as Message-ID, Date, and From.
- Trust the bounce: While Outlook might deliver the email, a bounce from Gmail indicates a genuine issue that needs addressing for broader deliverability. Bounce messages often contain specific error codes that can pinpoint the problem, such as the 552 5.7.0 error for duplicate headers.
- Check email formatting: Review your email client or sending platform's configuration for any settings that might inadvertently add duplicate headers or malform the message structure. Sometimes, issues related to DMARC, SPF, and DKIM configuration can also indirectly impact how headers are handled, so ensuring these are set up correctly is crucial. More information on email bounce back errors can be found here.
- Address specific error codes: For RFC5322 errors, specifically investigate issues like duplicate headers or malformed formatting that Gmail's systems are designed to detect more rigorously than Outlook's. Often, these errors might come with specific numeric codes, like 552 5.7.0, which point directly to header compliance problems. Learn more about fixing duplicate header errors in Gmail.
What email marketers say
Email marketers frequently encounter scenarios where their carefully crafted emails behave differently across various mailbox providers. The frustration of an email delivering perfectly to an Outlook inbox, only to be rejected by Gmail for RFC5322 compliance issues, is a common pain point. Marketers often look for quick fixes, but the underlying issue usually points to a fundamental problem with the email's structure or the sending system's adherence to standards.
Key opinions
- Inconsistent enforcement: Many marketers note that while their emails appear fine and are accepted by some providers (like Outlook), Gmail's stricter validation catches hidden issues.
- Perceived Gmail fault: There's a common initial reaction to blame Gmail for being too strict, rather than looking inward at their own sending practices.
- Header complexity: Understanding and debugging email headers can be daunting for marketers, often requiring technical assistance.
Key considerations
- Sender responsibility: It's critical for marketers to recognize that bounce errors, especially those citing RFC5322, point to an issue on their end, not the receiver's.
- Proactive header checks: Before sending large campaigns, marketers should use email testing tools to validate header compliance and avoid widespread bounce issues with providers like Gmail.
- Leverage diagnostics: Utilize Non-Delivery Reports (NDRs) to identify the specific nature of the RFC5322 error, which can often guide them to the exact malformed header. A general guide to email delivery errors explains these errors and their meanings. If you're encountering issues with Outlook specifically, there are resources for troubleshooting Outlook deliverability issues as well.
Marketer view
An email marketer from Email Geeks shared their observation that their emails are delivering successfully to Outlook.com accounts but are consistently bouncing when sent to Gmail addresses. They had already checked their email headers and were confident that there were no obvious issues, leading to confusion about Gmail's specific rejection criteria. This highlights the common challenge of inconsistent enforcement of email standards across different mailbox providers.
Marketer view
A marketer from a deliverability forum indicated they often see discrepancies in how various email clients handle malformed messages. They mentioned that while some providers might attempt to correct or overlook minor header issues, others, particularly Gmail, are very strict and will reject an email entirely if it does not strictly conform to RFC standards. This requires senders to be vigilant about their email generation process.
What the experts say
Email deliverability experts consistently highlight the critical importance of adhering to email standards like RFC5322 for successful inbox placement, particularly with providers known for their strict compliance checks. When Gmail bounces an email for RFC5322 errors while Outlook accepts it, experts agree that the problem almost invariably lies with the sender's email formatting, not with Gmail's filtering. This scenario underscores a key difference in how MTAs interpret and enforce RFC specifications.
Key opinions
- Sender is responsible: Experts emphasize that if Gmail rejects an email for RFC5322 compliance, the sending system is most likely at fault, not Gmail.
- Header duplication: A common cause for RFC5322 errors, specifically with Gmail, is the inclusion of duplicate headers which are forbidden by the standard.
- Differential enforcement: Outlook (and other providers) may be more tolerant of malformed headers or non-compliance than Gmail, leading to a false sense of security regarding email formatting.
Key considerations
- Thorough header analysis: Experts recommend meticulous examination of all email headers for proper formatting and uniqueness. Using a reliable email deliverability tester can provide detailed insights into your email's structure and compliance before it even leaves your system.
- Adhere to RFCs strictly: While some providers are lenient, designing emails to fully comply with RFC5322 and other relevant RFCs is crucial for universal deliverability. Further information on what RFC 5322 says versus what actually works is available.
- Trust bounce messages: Deliverability experts advise taking bounce messages seriously, even if the email delivered elsewhere. They are valuable diagnostic tools that indicate a genuine issue that needs fixing, such as the weird RFC 5322 bounces from Gmail discussed by Spam Resource.
Expert view
An expert from Email Geeks strongly advised that when an email bounces, especially for an RFC5322 error, the first step should always be to review the email headers. They stressed that it's highly improbable that a major provider like Google is at fault, and the issue almost certainly originates from how the sending system is constructing the email. This highlights the importance of sender-side compliance.
Expert view
A deliverability expert from Word to the Wise explained that while Outlook might forgive certain header violations, Gmail has a stricter interpretation of RFC5322 and related RFCs. They noted that issues like improper folding of long header lines or non-standard characters in specific header fields can trigger rejections from stricter receiving servers. This difference in parsing underscores the need for robust email generation.
What the documentation says
The Internet Engineering Task Force (IETF) and various RFCs (Requests for Comments) define the standards for email messaging, including the structure of email headers. RFC5322, specifically, details the format of Internet Message Format. Receiving mail servers like Gmail are designed to validate incoming emails against these specifications to ensure interoperability and to mitigate spam and abuse. When an email fails RFC5322 compliance, it's typically because a mandatory field is missing, a field is malformed, or a field that should appear only once is duplicated.
Key findings
- Header uniqueness: According to RFC5322, certain header fields (e.g., Message-ID, Date, From) MUST appear only once.
- Syntax validity: All header fields and their content must conform to specific ABNF (Augmented Backus-Naur Form) syntax rules defined within the RFC. Deviations lead to parsing errors.
- Mandatory fields: Emails require specific mandatory header fields to be present, such as Date, From, and Message-ID. Their absence will cause a compliance failure.
Key considerations
- Automated validation tools: Utilize automated tools that parse email headers and validate them against RFC5322 and other related RFCs. This is more reliable than manual inspection for complex issues.
- Strict vs. lenient parsers: Be aware that while all MTAs should ideally follow RFCs, some implement stricter parsing than others. Gmail is known to be one of the stricter implementers, causing issues for emails that might pass through more lenient systems like Outlook. Ensuring your DMARC records and policies are correctly configured is a critical step in building overall email deliverability. A list of DMARC tags and their meanings can provide further technical insight.
- Error message details: The Non-Delivery Report (NDR) or bounce message often contains specific error codes or phrases that directly point to the RFC5322 violation. Analyzing these details is paramount for diagnosis and resolution.
Technical article
Documentation from the IETF's RFC 5322, section 3.6, states that a message header must contain certain fields such as 'Date', 'From', and 'Message-ID', and that these fields must appear exactly once. It also specifies that fields like 'To', 'Cc', 'Bcc', and 'Subject' can appear at most once. This strict guidance is fundamental for email parsing and interoperability across different mail systems, emphasizing the importance of unique and correctly formatted mandatory headers.
Technical article
According to the official IETF RFC 5322 document, section 2.1.1, the basic structure of an email message consists of a header section followed by a body. It details that the header section is composed of field lines, each beginning with a field name, followed by a colon, and then the field body. Any deviation from this precise format, such as incorrect line endings or unexpected characters, renders the email non-compliant. This foundational structure is critical for proper parsing by receiving mail servers.
Related resources
2 resources
Related pages
How to troubleshoot email delivery issues related to RFC compliance errors?
Why am I getting a 5.7.1 delivery not authorized error in Gmail due to duplicate headers?
Why are emails suddenly rejected by Gmail?
Why are my emails having deliverability issues with Microsoft Outlook and Hotmail?
What RFC 5322 says vs. What Actually Works
A simple guide to DMARC, SPF, and DKIM
Why Your Emails Fail: Expert Guide to Improve Email Deliverability [2025]
Boost Email Deliverability Rates: Technical Solutions from Top Performing Senders
Understanding and troubleshooting DMARC reports from Google and Yahoo
