How to format messages according to RFC 5322 for Gmail sender guidelines?
Matthew Whittaker
Co-founder & CTO, Suped
Published 8 Jul 2025
Updated 17 Aug 2025
6 min read
Understanding how to format messages according to RFC 5322 has become increasingly important, especially with new Gmail sender guidelines. While the technical intricacies of RFC 5322 might seem daunting, especially if you are not a software developer or an MTA expert, adhering to its principles ensures your emails are correctly interpreted by recipient mail servers.
RFC 5322 defines the standard for the Internet Message Format, which is essentially the blueprint for how email messages should be structured. This includes everything from the various header fields to the body of the message. Without proper formatting, emails can be rejected, delayed, or even flagged as spam, impacting your overall deliverability.
For most senders using a reputable Email Service Provider (ESP), much of this technical compliance is handled automatically. However, knowing the basics can help you troubleshoot issues and understand the underlying reasons for certain deliverability challenges.
The fundamentals of RFC 5322 messaging
RFC 5322, formally known as the Internet Message Format, is the foundational standard that outlines the exact syntax for email messages. It dictates how headers, such as From, To, Subject, and Date, should be constructed, and also covers the structure of the message body. This adherence ensures interoperability across different email systems worldwide.
Key elements defined in RFC 5322 include the message header and the message body. The header consists of various fields, each with a specific syntax and purpose. For instance, the From field must contain a valid email address, and the Date field must follow a specific format. The body contains the actual content of the email.
Additionally, the standard specifies rules for character sets and line endings. While it recommends US-ASCII character encoding, it also permits other encodings like UTF-8. Crucially, lines of text in the email message should not exceed 998 characters, including the carriage return and line feed characters, and should be properly terminated with CRLF (carriage return and line feed).
Gmail's emphasis on RFC 5322 compliance
Gmail (and Yahoo) have recently updated their sender guidelines, explicitly stating the need to format messages according to RFC 5322. This requirement is a foundational step towards improving email security and reducing spam. Mailbox providers expect well-formed messages that adhere to these standards to prevent misinterpretation or malicious exploitation of email structures.
For Gmail, specific headers are particularly critical. Every message must include a valid Message-ID header, which uniquely identifies each email. Other essential headers like Date, From, To, and Subject must also be correctly formatted and present. Failure to do so can result in delivery issues.
Gmail's strictness
Gmail’s systems are designed to identify and flag messages that deviate from RFC 5322. This includes issues like duplicate header fields, incorrect character encoding, or malformed email addresses in the From header. Messages with such errors are often treated as suspicious, leading to them being sent to spam folders or rejected entirely. This is part of their broader effort to combat email abuse and enhance user security.
Common pitfalls in email formatting
While most modern ESPs handle RFC 5322 compliance automatically, some common pitfalls can still occur, especially in custom setups or when migrating systems. One frequent issue is exceeding the maximum line length of 998 characters. This often happens with long URLs or base64 encoded images that are not properly folded onto multiple lines.
Another common mistake is the duplication of header fields that should only appear once, such as From, To, or Subject. Incorrect or unusual character encoding, or using abnormal characters in email addresses, can also lead to compliance issues and potential deliverability problems.
Common problems
Improper line breaks within the email body or headers, especially in HTML emails, can lead to RFC 5322 non-compliance. This often occurs when email content is generated dynamically without strict adherence to line length limits, causing messages to appear malformed or be rejected by receiving servers.
Failure to include a valid and unique Message-ID header for each message can result in emails being dropped or marked as spam. This identifier is crucial for tracking and threading emails across different systems.
Solutions
Ensure your sending system or ESP automatically folds long lines in headers and the body, breaking them at appropriate points. For HTML content, ensure that your HTML is well-formed and does not interfere with the underlying message format.
Verify that your email sending platform automatically generates and includes a unique Message-ID for every email sent. This is a fundamental requirement for reliable email delivery.
How your ESP handles RFC 5322
For the vast majority of email senders, RFC 5322 compliance is not a concern because reputable ESPs handle this automatically. When you compose an email using their platform, the underlying message formatting, including proper headers, line endings, and character encoding, is managed by the ESP's infrastructure. This means you can focus on your content and audience, rather than worrying about the technical standards.
However, if you are running your own mail server, developing a custom sending solution, or integrating a system directly with an MTA, then strict adherence to RFC 5322 becomes your responsibility. In these cases, even minor deviations can lead to messages being rejected by major mailbox providers like Gmail. If you're encountering bounces related to RFC 5322 issues, such as Gmail corporate addresses bouncing, it usually points to an underlying configuration problem with your sending software.
For example, a common issue for non-compliant emails is malformed header fields. Here’s an example of a good versus a bad From header:
Good From: header formats
From: Sender Name <sender@example.com>
From: "Sender Name" <sender@example.com>
Bad From: header formats
From: Sender Name <<sender@example.com>>
From: sender@example.com Sender Name
Troubleshooting and maintaining compliance
While RFC 5322 compliance is a technical requirement, its direct impact on deliverability is often overshadowed by other factors like sender reputation, email authentication (SPF, DKIM, DMARC), and content quality. If your emails are bouncing or landing in spam folders due to RFC 5322 errors, you might see specific bounce messages indicating issues with message formatting. For instance, you might encounter Gmail bouncing emails with RFC 5322 errors, even if Outlook accepts them.
Troubleshooting RFC 5322 issues typically involves reviewing your email sending logs for specific error codes or messages. If you are using an ESP, contact their support to verify their compliance and inquire about any known issues. For self-managed systems, you would need to inspect the raw email source to pinpoint the exact formatting error. Remember, adhering to Gmail's new sending rules is crucial for maintaining good deliverability.
Views from the trenches
Best practices
Ensure your Email Service Provider (ESP) is reputable and automatically handles RFC 5322 compliance, as this is usually not a user-level concern.
Verify that unique Message-ID headers are generated for every email sent, a key RFC 5322 requirement for deliverability.
Pay attention to email line length limits, ensuring no line exceeds 998 characters, including CRLF, to prevent formatting issues.
Always use standard email character encodings like UTF-8 for message content to ensure proper display and avoid parsing errors.
Implement robust email authentication protocols such as SPF, DKIM, and DMARC, which are critical for sender reputation alongside RFC compliance.
Common pitfalls
Duplicating header fields (e.g., 'From', 'To', 'Subject') that should appear only once, leading to RFC 5322 violations.
Allowing email content, especially long URLs or base64 data, to create lines exceeding 998 characters without proper folding.
Using abnormal or non-standard characters in email addresses or headers, which can trigger errors or rejections by mail servers.
Relying solely on RFC 5322 compliance without also addressing sender reputation, content quality, or email authentication.
Assuming your custom-built email sending system automatically handles all RFC 5322 nuances without thorough testing and validation.
Expert tips
If building a custom sending solution, use well-tested email libraries and frameworks designed to ensure RFC compliance.
Regularly monitor your email sending logs for any bounce messages specifically mentioning RFC 5322 or formatting errors.
Prioritize a clean and engaged mailing list; poor list hygiene impacts deliverability far more than minor RFC deviations.
Leverage Google Postmaster Tools and DMARC reports to gain insights into how your emails are being received and processed.
If experiencing deliverability issues, first focus on authentication, content, and sender reputation before diving deep into RFC 5322.
Expert view
Expert from Email Geeks says that unless you are a Mail Transfer Agent (MTA) developer, most email senders do not need to concern themselves with the technicalities of RFC 5322 compliance, as ESPs typically handle this automatically.
2024-03-13 - Email Geeks
Marketer view
Marketer from Email Geeks says that ensuring lines do not exceed 998 characters, including the carriage return and line feed, is crucial, especially since HTML tags do not count as line breaks and physical returns are necessary.
2024-03-13 - Email Geeks
Key takeaways for reliable email delivery
While RFC 5322 compliance is a fundamental requirement for email deliverability, especially with new Gmail's sender guidelines, it is largely managed behind the scenes by reliable ESPs. For most senders, focusing on broader deliverability best practices, such as maintaining a good sender reputation, implementing proper email authentication (SPF, DKIM, DMARC), and sending engaging, relevant content, will yield far greater results.
If you are experiencing deliverability issues, it's more likely related to these other factors than to RFC 5322 formatting. However, understanding the basic tenets of email formatting can still be beneficial for troubleshooting and ensuring your email program is robust. Always leverage the tools and support provided by your ESP or email security platform to monitor and improve your inbox placement.
Gmail. If you're encountering bounces related to RFC 5322 issues, such as Gmail corporate addresses bouncing, it usually points to an underlying configuration problem with your sending software.
