Why are Gmail emails bouncing with '553 5.1.3 The recipient address is not a valid RFC-5321 address' error?
Matthew Whittaker
Co-founder & CTO, Suped
Published 1 Jul 2025
Updated 19 Aug 2025
8 min read
Encountering a '553 5.1.3 The recipient address is not a valid RFC-5321 address' bounce error from Gmail can be a frustrating experience for anyone managing email campaigns or transactional sends. This specific error indicates that the recipient email address, as presented to Gmail's mail servers, does not conform to the Simple Mail Transfer Protocol (SMTP) standard defined in RFC 5321.
While the message itself is quite direct, pinpointing the exact cause can sometimes be tricky. It often boils down to subtle formatting issues, such as extraneous spaces or prohibited characters, within the recipient's email address. Understanding the nuances of the RFC 5321 standard is key to diagnosing and resolving these bounce errors effectively.
This guide will delve into what RFC 5321 entails for email addresses, explore the most common culprits behind this particular Gmail bounce, and provide actionable steps to troubleshoot and prevent them. We'll examine how these errors impact your email deliverability and how you can maintain a clean and compliant email list.
Understanding the RFC 5321 standard
RFC 5321, formally known as the Simple Mail Transfer Protocol (SMTP), defines the rules for how email messages are transmitted across the internet. It specifies the commands and replies that email servers use to communicate, including how they identify senders and recipients. When Gmail returns a '553 5.1.3' error, it's explicitly stating that the recipient address provided during the RCPT TO command in the SMTP conversation does not adhere to these fundamental standards.
The RFC 5321 specification for email addresses is quite strict. It dictates valid characters, structure, and length. For instance, while it allows for a wide range of characters, certain symbols have specific meanings or are outright prohibited in certain parts of the address. The crucial point here is that the format must be unambiguous for the receiving server to correctly parse and route the email. You can find more details on the official RFC 5321 documentation.
This standard is different from RFC 5322, which governs the format of the email message header and body. While related, the '553 5.1.3' error specifically points to the SMTP envelope recipient, not issues with the message content or headers. For issues related to RFC 5322 errors, a different approach is typically needed. Gmail's stringent validation helps prevent malformed addresses from clogging up its systems and aids in maintaining high email deliverability across its network.
Common causes of the 553 5.1.3 error
The most frequent reason for this error is often subtle formatting mistakes in the recipient's email address. Even a single extra space can invalidate an address according to Gmail's interpretation of RFC 5321. This can be particularly frustrating because such errors often go unnoticed in basic validation checks.
Common culprits include leading or trailing spaces, as well as invalid special characters. For example, some systems might mistakenly add a space before or after the address when storing or retrieving data, or a database migration could introduce an encoding issue that results in non-standard characters appearing in the address. This is why it's crucial to check how addresses are stored and handled throughout your email sending process.
Valid RFC 5321 addresses
Standard Format: User@example.com (no leading/trailing spaces)
Plus Addressing: user+tag@example.com (used for filtering)
Subdomains: user@sub.domain.com
Invalid RFC 5321 addresses (Common Issues)
Trailing/Leading Spaces: " user@example.com" or "user@example.com "
Prohibited Special Characters: user”name@example.com (curly quotes)
Missing Domain Part: user@
Sometimes, the issue isn't with how you store the address, but with how your email service provider (ESP) or internal system processes it before sending. They might be inadvertently adding characters or failing to strip invalid ones. This can lead to a situation where emails previously delivered successfully suddenly start bouncing, as Gmail (or any receiving server that strictly adheres to the RFC) begins enforcing stricter validation rules. In some cases, a general increase in SMTP errors from Gmail could indicate changes on their end that require your sending infrastructure to be more compliant. For additional troubleshooting on why emails are rejected by Gmail, review other potential causes as well.
Troubleshooting and resolving the bounce
The first step in troubleshooting is to isolate where the invalid format is being introduced. Begin by checking your raw data source, such as your CRM or database, to see how the problematic email addresses are stored. Look for any inconsistencies or unusual characters. If the addresses appear correct there, the issue might lie in the data export or the email sending platform itself.
Next, examine the logs from your email sending system or ESP. These logs often provide the exact recipient address as it was presented to the receiving mail server. You might find that a seemingly invisible character, like a non-breaking space or a curly quote (” instead of "), is present. These can be particularly insidious because they are not always visually obvious.
One effective way to test how Gmail handles specific recipient addresses is to perform a direct SMTP test using telnet or a similar command-line tool. This allows you to simulate the exact SMTP conversation and see the error message firsthand. Notice how Gmail explicitly rejects the address with the leading space in the example below.
SMTP test with telnet showing RFC 5321 errorBASH
telnet gmail-smtp-in.l.google.com 25
Trying 2a00:1450:400b:c01::1a...
Connected to gmail-smtp-in.l.google.com.
Escape character is '^]'.
220 mx.google.com ESMTP l188si3980706wmf.134 - gsmtp
HELO blighty.com
250 mx.google.com at your service
MAIL FROM:<steve@blighty.com>
250 2.1.0 OK l188si3980706wmf.134 - gsmtp
RCPT TO:<wttwsteve@gmail.com>
250 2.1.5 OK l188si3980706wmf.134 - gsmtp
RCPT TO:< wttwsteve@gmail.com>
553-5.1.3 The recipient address < wttwsteve@gmail.com> is not a valid RFC-5321
553 5.1.3 address. l188si3980706wmf.134 - gsmtp
quit
221 2.0.0 closing connection l188si3980706wmf.134 - gsmtp
Connection closed by foreign host.
If you're dealing with a sudden surge of these errors, and addresses previously worked, it's worth communicating with your ESP. They might have recently updated their email parsing or validation routines, or there could be a bug on their end introducing these formatting issues. Always ensure your technical solutions are up to date to mitigate such problems. For other issues with Gmail accounts bouncing, consult other related guides.
Preventing future 553 5.1.3 errors
The best defense against '553 5.1.3' errors is proactive data hygiene and validation. Implement robust validation checks at the point of data entry and before sending any emails. This should include trimming whitespace and validating against a regular expression that adheres to RFC 5321 (and realistically, the stricter RFC 5322) standards.
Regularly clean your email lists. Remove any addresses that consistently bounce with this error, or any other type of hard bounce (e.g., 'user unknown' or 'domain does not exist'). Maintaining a clean list not only reduces bounce rates but also improves your sender reputation, which is critical for overall email deliverability. Pay attention to any 'blocklist' (or blacklist) notifications you receive, as consistently sending to bad addresses can lead to your email address ending up on a blacklist.
Consider implementing a confirmation step for new email sign-ups, often called double opt-in. This verifies that the email address is not only valid but also actively monitored by a real person. This practice significantly reduces the chances of invalid addresses entering your system. While email standards can be complex, adopting strict adherence to them ensures smooth delivery. For advanced email authentication methods, you might explore DMARC, SPF, and DKIM.
Views from the trenches
Best practices
Implement stringent input validation and sanitization for all email addresses at the point of collection.
Regularly audit your email list for malformed or invalid addresses and clean them.
Utilize an email validation service before sending campaigns to catch non-compliant addresses.
Ensure your email service provider (ESP) or sending platform is correctly handling email addresses without introducing extra characters or spaces.
Common pitfalls
Overlooking invisible characters like leading/trailing spaces or non-standard special characters.
Relying solely on basic client-side or server-side validation that doesn't check RFC compliance.
Failing to monitor bounce logs specifically for RFC-related errors, which can indicate deeper issues.
Assuming that if an email address worked before, it will always remain valid without re-validation.
Expert tips
Verify how your system's database stores email addresses, as encoding issues can introduce problems.
Use a tool that can perform an SMTP-level test to precisely replicate how receiving servers see your addresses.
Review your ESP's documentation or support for common issues related to address formatting and compliance.
When integrating new systems, ensure email address fields are mapped correctly and sanitized during transfer.
Marketer view
Marketer from Email Geeks says they previously had a similar issue and discovered that special characters in the 'from' name, rather than the address itself, were causing problems.
2021-01-14 - Email Geeks
Expert view
Expert from Email Geeks says checking how email addresses are stored is the first step when encountering specific rejection messages like this one.
2021-01-15 - Email Geeks
Ensuring valid email delivery
The '553 5.1.3 The recipient address is not a valid RFC-5321 address' error from Gmail is a clear signal that the email address format being used does not meet the established internet standards for email communication. While seemingly minor, issues like hidden spaces or illegal characters can severely impact your email campaigns and overall sender reputation.
By understanding the strict requirements of RFC 5321 and implementing rigorous data validation and list hygiene practices, you can significantly reduce the occurrence of these bounces. Regularly auditing your data, leveraging proper validation tools, and maintaining open communication with your ESP are essential steps in ensuring your emails consistently reach their intended recipients.
Prioritizing the accuracy and compliance of your email addresses will not only resolve this specific bounce error but also contribute to a healthier, more effective email program overall, leading to improved email deliverability and engagement rates.
Gmail can be a frustrating experience for anyone managing email campaigns or transactional sends. This specific error indicates that the recipient email address, as presented to Gmail's mail servers, does not conform to the Simple Mail Transfer Protocol (SMTP) standard defined in RFC 5321.
