Summary
The Gmail bounce error "553 5.1.3 The recipient address is not a valid RFC-5321 address" indicates that the email address format used for the recipient does not comply with the specifications outlined in RFC 5321, which defines the Simple Mail Transfer Protocol (SMTP). This error commonly arises due to subtle formatting issues, such as leading or trailing spaces, or other non-standard characters within the email address. While your ESP or internal systems might have previously cleaned these addresses, Gmail has tightened its enforcement of these standards, leading to new bounces. For related issues, you might also find our guide on RFC 5322 errors helpful, as these standards are closely linked.
Key findings
- Specific error: The "553 5.1.3" bounce message from Gmail explicitly states that the recipient email address provided is not a valid RFC 5321 address.
- Common culprits: The primary cause is often invisible characters, such as leading or trailing spaces, within the recipient email address string.
- Recent enforcement: Gmail may have recently begun stricter enforcement of RFC 5321 syntax, causing previously accepted, but malformed, addresses to now bounce. For broader context on rejections, see our article about why emails are rejected by Gmail.
- Source of error: The issue likely originates from how the email addresses are stored in your database or processed by your sending system, rather than a widespread Gmail system problem. This is consistent with what Xojo forum users noted about checking the value being passed.
Key considerations
- Data hygiene: Regularly validate and clean your email lists to remove any addresses that do not conform to RFC standards.
- Input validation: Implement client-side and server-side validation to prevent malformed email addresses from entering your system.
- System review: Investigate your database storage, CRM, and ESP (Email Service Provider) integrations to identify where the invalid characters are being introduced or are failing to be removed. As discussed on the concrete5.org forum, this often involves checking the 'to' parameter.
- Pre-send sanitization: Even if your ESP typically cleans addresses, a recent change in their processing or Gmail's requirements might necessitate your own pre-send sanitization.
What email marketers say
Email marketers experiencing the "553 5.1.3" bounce error from Gmail primarily suspect issues with how recipient email addresses are stored and handled within their systems or by their ESPs. Many point to the possibility that these systems previously cleaned malformed addresses, but recent changes in Gmail's validation (or the ESP's processing) are now exposing underlying data hygiene problems. They advise checking for subtle formatting errors, such as hidden spaces, and exploring their own platforms before assuming a widespread Gmail issue. This highlights the importance of rigorous email list management, a topic we cover in more detail in our article on improving email deliverability.
Key opinions
- Internal data issues: Many marketers believe the problem lies in their own data storage, where recipient addresses might have hidden leading or trailing spaces. This is consistent with how email accounts bounce with 'No Such User' errors if the address itself is incorrect.
- Recent behavioral change: There's a shared observation that emails to these problematic addresses were previously delivering fine, suggesting a recent shift in Gmail's acceptance criteria or the ESP's handling.
- ESP interaction: Marketers frequently question whether their ESP is introducing the invalid characters or, conversely, has stopped cleaning them before sending.
- From name characters: Some users have experienced similar issues due to special characters in the 'from name' field, not just the 'to' address, leading to RFC compliance problems.
Key considerations
- Examine data sources: Thoroughly check how email addresses are entered, stored, and retrieved from your CRM or database. Implement trimming functions to remove leading/trailing spaces.
- Liaise with ESP: Contact your ESP's support to inquire about any recent changes in their email address validation or sanitization processes that might explain the new bounces.
- Test problematic addresses: Isolate a few of the bouncing email addresses and manually test sending to them, ensuring they are copied exactly from your system to replicate the issue. You can use an online email deliverability tester for this.
- Review 'from' header: If the recipient address appears clean, investigate the 'from name' and other headers for any unusual or non-standard characters that could be causing RFC non-compliance.
Marketer view
Email marketer from Email Geeks suggests checking how email addresses are currently stored, as this is a very specific rejection message indicating a formatting issue.
Marketer view
Forum user from concrete5.org indicates that the error clearly points to an invalid recipient address, urging a check of the value being passed in the 'to' parameter of the email client or library.
What the experts say
Email deliverability experts agree that the "553 5.1.3" Gmail bounce error is a clear indicator of a recipient address formatting issue that violates RFC 5321. They emphasize that such a specific error message points to a problem with the sender's data or system, rather than a widespread Gmail outage. Historically, ESPs or Gmail itself might have been more lenient or applied automatic corrections, but modern systems enforce stricter adherence to RFC standards. Experts strongly recommend a thorough review of data storage and transmission processes to identify and rectify where invalid characters or spaces are introduced. This kind of bounce is distinct from a general user unknown bounce, focusing specifically on formatting.
Key opinions
- Sender-side problem: The error is almost certainly on the sender's side, indicating an issue with how the email addresses are stored or processed, not a global Gmail problem.
- Specific formatting: The error message is highly specific to RFC 5321 address formatting, particularly flagging issues like leading or trailing spaces.
- Telnet confirmation: Testing with a direct SMTP connection (like telnet) can confirm that Gmail rejects addresses with spaces, proving the server's strict adherence to RFCs.
- Historical leniency: It is plausible that either the ESP or Gmail was previously more tolerant of invalid addresses, or actively cleaning them, which has now ceased.
Key considerations
- Root cause analysis: Pinpoint where the invalid characters or spaces are introduced into the email addresses within your system. This requires checking data entry, storage, and transfer layers.
- Automated cleaning: Implement robust automated cleaning routines that trim whitespace and validate email address syntax before sending.
- Email validation: Use an RFC-compliant email validation service or library to pre-emptively identify and correct malformed addresses before they lead to bounces. For more on this, check out our Email Deliverability Tester.
- Monitor system changes: Be aware that email standards enforcement can evolve, and what was once tolerated by recipient servers might not be in the future. Experts at Word to the Wise often discuss these changes.
Expert view
Expert from Email Geeks states that the rejection message is highly specific, indicating that the first step should be to meticulously examine how the email addresses are stored in the system.
Expert view
Expert from SpamResource.com notes that syntax errors in email addresses, particularly those related to spaces or invalid characters, are unambiguous violations of RFC 5321 and will result in hard bounces.
What the documentation says
RFC 5321 (Simple Mail Transfer Protocol) is fundamental to email communication, explicitly defining the syntax and rules for email addresses in SMTP commands like RCPT TO. The error "553 5.1.3 The recipient address is not a valid RFC-5321 address" directly refers to a violation of these established standards. Documentation typically stresses the importance of adherence to these specifications for reliable email delivery. Deviations, even subtle ones like extra whitespace, render an address non-compliant and are grounds for rejection by mail servers. This contrasts with some aspects of RFC 5322, which focuses on message format, although both RFCs are vital for overall email compliance and deliverability, as detailed in our article What RFC 5322 says.
Key findings
- Strict syntax: RFC 5321 mandates a precise syntax for email addresses within SMTP commands, disallowing leading/trailing spaces or other invalid characters.
- Recipient address validation: Mail servers (like Gmail's) are expected to validate the syntax of the recipient address presented in the RCPT TO command against RFC 5321.
- Immediate rejection: Non-compliance with RFC 5321 for the recipient address typically results in an immediate 553 5.1.3 bounce error, as the address is fundamentally malformed.
- Distinction from other errors: This error is distinct from issues like the recipient mailbox being full (which might yield a 552 error), or content-related rejections.
Key considerations
- Automated validation: Systems sending email should include robust automated processes to validate email addresses against RFC 5321 syntax before initiation of the SMTP conversation.
- Data cleansing: Regular data cleansing is necessary to remove any email addresses that may have accumulated invalid characters or formatting over time.
- Protocol adherence: Any platform or application that constructs email addresses for sending should strictly adhere to the address syntax defined in RFC 5321. This also applies to the 'from' address, which can lead to errors like abnormal characters in sender's email.
- Software updates: Ensure that your email sending software, libraries, and ESP configurations are up-to-date and compliant with current RFC standards. You can refer to the official RFC 5321 specification for precise details.
Technical article
The RFC 5321 specification outlines the precise syntax required for email addresses in the RCPT TO command, explicitly stating that any deviation, including the presence of whitespace, renders the address invalid.
Technical article
Google's postmaster guidelines specify that recipient addresses must conform to internet standards for proper delivery, and non-compliance will lead to rejections like the 553 5.1.3 error.
Related resources
2 resources
Related pages
Why is Gmail bouncing my emails with RFC5322 errors when Outlook accepts them?
What causes '550 5.1.1 Recipient address rejected: User unknown' email bounce errors?
Why are emails suddenly rejected by Gmail?
What causes Gmail's 'sender's email address uses abnormal characters' error?
What RFC 5322 says vs. what actually works
Email Deliverability Issues: Getting Your Messages to the Inbox in 2025
Why Your Emails Fail: Expert Guide to Improve Email Deliverability [2025]
A simple guide to DMARC, SPF, and DKIM
