What special characters are allowed in email address syntax according to RFC 5322 and how do different email providers handle them?

Summary

While RFC 5322 defines email address syntax, including allowed 'atext' characters and the use of quoted strings, practical implementation varies across providers. Many providers enforce stricter rules than the RFC, disallowing certain special characters, particularly at the beginning or end of the local part, or consecutive periods. Gmail ignores periods in the local part. Yahoo often rejects addresses with uncommon special characters despite RFC validity. Validation libraries may be overly restrictive. Local parts have a 64-character limit, domains 255, and the entire address 320. Invalid characters can cause hard bounces, damaging sender reputation. Deliverability hinges on provider policies, and less common special characters might trigger spam filters. International characters should be avoided due to deliverability issues. Therefore, email address validation is crucial for maintaining a clean list, improving deliverability, and avoiding negative impacts on campaigns and sender reputation. Blanket rules should only adhere to RFC guidelines.

Key findings

  • RFC vs. Implementation: RFC 5322 defines the standard, but email providers often deviate in their implementation.
  • Atext and Quoted Strings: RFC defines specific atext characters and allows any character within quoted strings.
  • Provider-Specific Rules: Providers implement stricter rules, such as disallowing specific characters or character combinations.
  • Gmail Period Handling: Gmail ignores periods in the local part of email addresses.
  • Character Rejection: Yahoo rejects addresses with unusual special characters, even if technically valid.
  • Validation Issues: Validation libraries can be overly restrictive and reject valid addresses.
  • Character Limits: Local parts have a 64-character limit, domains have a 255-character limit and total email address length is limited to 320 characters.
  • Hard Bounces: Invalid characters can lead to hard bounces and damage sender reputation.
  • Deliverability Impact: Deliverability depends on provider policies and can be affected by special characters.
  • International Characters: Avoid international characters due to potential deliverability problems.

Key considerations

  • Email Validation: Implement robust email validation to ensure valid addresses are used.
  • Provider Awareness: Understand and adapt to provider-specific restrictions on email address syntax.
  • Test Thoroughly: Thoroughly test email validation methods to ensure compatibility with major providers.
  • Monitor Performance: Monitor email deliverability and bounce rates to identify potential issues.
  • Base Rules: Set blanket rules that align to RFC guidelines.

What email marketers say
12Marketer opinions

While RFC 5322 defines the syntax for valid email addresses, including which special characters are permitted, email providers often implement stricter rules. Many providers disallow certain special characters, particularly at the beginning or end of the local part, or consecutive periods. Gmail ignores periods in the local part. Yahoo often rejects addresses with uncommon special characters, despite RFC 5322. Validation libraries are often overly restrictive. The local part of the email address has a maximum length of 64 characters, the domain has a limit of 255 characters, and the entire email address can be no more than 320 characters. Using invalid characters can cause hard bounces, damaging sender reputation. Therefore, email address validation is crucial for maintaining a clean list, improving deliverability, and avoiding negative impacts on campaigns and sender reputation.

Key opinions

  • RFC 5322 vs. Reality: RFC 5322 defines the standard, but email providers often have stricter rules.
  • Character Restrictions: Many providers restrict special characters allowed in the local part of the address.
  • Gmail's Period Handling: Gmail ignores periods in the local part of an email address.
  • Validation Inaccuracies: Many validation libraries are overly restrictive and may reject valid addresses.
  • Length Limits: The local part has a 64 character limit, the domain 255, and the total address 320.
  • Impact of Invalid Addresses: Using invalid characters can lead to hard bounces and damage sender reputation.

Key considerations

  • Email Validation: Implement robust email validation processes to ensure valid addresses are used.
  • Provider-Specific Rules: Be aware of provider-specific restrictions on special characters.
  • Testing and Monitoring: Regularly test email validation methods and monitor bounce rates to identify potential issues.
  • Use up-to-date Validation: Use validation libraries that are regularly updated with latest provider rules.
Marketer view

Marketer from Email Geeks explains that as per RFC 5322, section 3.2.3, the provided email address syntax is valid. The definition of “atext” lists the allowed characters, as clarified in section 3.4.1.

January 2022 - Email Geeks
Marketer view

Email marketer from ZeroBounce states that email address validation helps ensure that you only send emails to real users and avoids sending emails to invalid addresses which can harm your sender reputation.

January 2023 - ZeroBounce

What the experts say
7Expert opinions

Email address validity exists on two levels: syntax and provider acceptance. While RFC specifications outline permitted characters, deliverability hinges on individual mailbox provider policies and filtering rules. Any character is valid within a double-quoted string, while others are only valid as 'atext'. The local part is constructed of atoms separated by periods or a double-quoted string, requiring awareness of atom rules. Maintaining provider-specific rules is challenging due to frequent changes. Using less common special characters can trigger spam filters, despite technical validity. International characters should be avoided due to deliverability and technical problems. Blanket rules should align with RFC guidelines, and not more.

Key opinions

  • Validity Levels: Email address validity depends on syntax and provider acceptance.
  • Deliverability vs. Syntax: Valid syntax doesn't guarantee deliverability due to provider policies.
  • Character Rules: Characters inside a double quoted string are valid, others must follow 'atext' rules.
  • Local Part Construction: Local part is atoms separated by periods or a double-quoted string requiring awareness of atom rules.
  • International Characters: International characters can cause deliverability and technical issues, and should be avoided.
  • RFC Alignment: Blanket rules should align with RFC guidelines.

Key considerations

  • Provider Policies: Understand and adapt to individual mailbox provider policies.
  • Spam Filtering: Avoid less common special characters that may trigger spam filters.
  • Complex Rules: Be aware of the complex rules governing the local part, including atom rules.
  • String Handling: Handle double quoted strings correctly to ensure validity of certain characters.
Expert view

Expert from Word to the Wise, Laura Belgray, responds to a user question about allowing international characters. It states that you should avoid these characters because they will cause deliverability and technical issues.

July 2021 - Word to the Wise
Expert view

Expert from Email Geeks clarifies that valid syntax doesn't guarantee deliverability and maintaining a list of rules enforced by each MX provider is an attempt to reverse engineer something that changes, the usefulness of which depends on why one wants to know.

May 2022 - Email Geeks

What the documentation says
3Technical articles

RFC 5322 specifies allowed characters in 'atext', including alphanumeric and several special characters like `!`, `#`, etc. It also permits any character within a quoted string in the local part. However, practical implementation by email providers often deviates from the RFC standard, with some providers not supporting all allowed characters, especially special characters in the local part.

Key findings

  • RFC 5322 'atext': RFC 5322 defines specific alphanumeric and special characters allowed in 'atext'.
  • Quoted Strings: Any character is allowed within a quoted string in the local part of an email address according to RFC 5322.
  • Implementation Variance: Email providers' implementation often differs from RFC 5322, limiting support for certain characters.

Key considerations

  • Provider Compatibility: Consider email providers' specific character support when designing email systems.
  • Validation Testing: Thoroughly test email address validation to ensure compatibility with major providers.
  • RFC as Baseline: Use RFC 5322 as a baseline understanding of the standard but recognize its limitations in real-world applications.
Technical article

Documentation from ietf.org clarifies that RFC 5322 allows any character within a quoted string in the local part of an email address. However, the implementation of quoted strings may vary among different email providers.

July 2021 - ietf.org
Technical article

Documentation from Wikipedia outlines that while RFC 5322 defines the standard for email address syntax, practical implementation often differs. Some providers may not support all characters allowed by the RFC, particularly special characters in the local part.

January 2024 - Wikipedia