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

Key findings

• RFC compliance: RFC 5322, specifically sections 3.2.3 and 3.4.1, permits a wide array of special characters in the local part (the segment before the '@' symbol) of an email address. This includes characters like !, #, %, &, ^, and ~, among others.
• Local part structure: The local part of an email address can be structured as one or more 'atoms' separated by periods, or it can be a 'double-quoted string'. Within a double-quoted string, almost any character is permissible, provided it is properly escaped.
• Provider variance: Despite RFC specifications, many major email providers like Gmail and Yahoo impose stricter, more pragmatic rules on allowed characters during address registration and for deliverability purposes. These limitations often extend beyond simple syntax checks.
• Functional vs. technical validity: An email address can be technically valid according to RFC 5322 but may not be deliverable or usable on specific platforms, as email providers prioritize deliverability and abuse prevention over strict RFC adherence.

Key considerations

• Validation strategy: While RFC 5322 defines the broadest possible syntax, your validation strategy should balance this with the practicalities of deliverability. Overly strict validation can reject legitimate addresses, while overly permissive validation can lead to high bounce rates.
• Provider-specific nuances: Be aware that providers handle special characters differently. For instance, Gmail effectively ignores dots in the local part and supports plus addressing for filtering. This means test.user@gmail.com and testuser@gmail.com are often treated as the same address. See how Gmail handles dots.
• Email validation tools: Employ robust email validation tools that go beyond basic regex checks. These tools often simulate sending to determine actual deliverability, taking into account provider-specific rules and common blacklist practices.
• Maintainability: Attempting to reverse engineer and maintain a comprehensive list of character restrictions for every MX provider is impractical due to frequent changes in their policies. Instead, focus on generally accepted best practices for email address structure.

Key opinions

• RFC vs. reality: Marketers frequently encounter addresses that are RFC 5322 compliant but fail validation or delivery when interacting with popular email services, illustrating a gap between theoretical standards and practical application.
• Provider registration limits: It's observed that providers like Yahoo or Gmail often restrict the characters allowed when a user creates an email address, even if those characters are technically valid under RFC 5322. For example, a / character might be rejected at signup.
• Deliverability focus: The primary concern for marketers is whether an email address will reliably receive messages, not just its RFC validity. This pushes them towards more conservative validation rules.
• Impact of special characters: Some special characters, while RFC-compliant, might trigger spam filters or be viewed suspiciously by receiving servers, affecting inbox placement.

Key considerations

• Balancing strictness: Marketers should avoid overly restrictive validation rules that prevent legitimate users with RFC-valid addresses from signing up. It's important to consider that a range of characters are allowed.
• Database hygiene: Regular email list cleaning and validation are essential to manage addresses that might be technically valid but functionally undeliverable, impacting overall email program performance.
• Sender reputation impact: Sending to invalid or problematic addresses can negatively affect your sender reputation. It is important to focus on valid addresses.
• Evolution of rules: Marketers should stay informed about evolving email provider guidelines and features, such as how Gmail handles dots or plus addressing, as these can affect how special characters are perceived and processed.

Key opinions

• Syntax vs. deliverability: Experts strongly differentiate between an email address being RFC-compliant and it being truly deliverable. Just because the syntax is correct doesn't mean a message sent to it will reach the inbox.
• Provider-specific interpretation: Mailbox providers (like Gmail, Outlook) often apply their own interpretations and additional restrictions beyond the RFC for various reasons, including spam prevention and usability. This means they might reject addresses that are technically valid per RFC 5322.
• Impact of strict validation: Overly strict email validation at the sender's end, which deviates from RFC in a restrictive way, can lead to blocking legitimate users. For more on this, check out what RFC 5322 says versus what actually works.
• Dynamic nature: Attempting to create and maintain an exhaustive list of rules for every provider is futile, as these policies are subject to change and are often proprietary. This implies that relying solely on such lists is not a sustainable strategy.

Key considerations

• Pragmatic approach: Adopt a pragmatic approach to email validation that prioritizes deliverability over strict RFC adherence where necessary. This often involves using a blend of RFC checks and real-world deliverability tests.
• Double-quoted strings: While RFC 5322 permits almost any character within a double-quoted string in the local part, experts caution that many common email systems may not handle these well, leading to bounces or delivery errors.
• Plus addressing and periods: Understand how providers manage features like plus addressing (e.g., user+tag@example.com) and ignore periods in Gmail addresses. These behaviors impact how duplicate addresses are perceived and managed in your lists. For more information, read safe email validation.
• Avoiding over-validation: Experts advise against implementing overly complex regular expressions for email validation, as they frequently fail to capture the full spectrum of RFC-compliant addresses and can lead to unintended rejections.

Key findings

• Core definition: RFC 5322 specifies the Internet Message Format (IMF), which includes the syntax for email addresses, broken down into a local-part@domain-part structure.
• Atext characters: The RFC defines 'atext' as a set of characters permitted in atoms within the local part. These include uppercase and lowercase Latin letters (A-Z, a-z), digits (0-9), and various printable characters such as !, #, $ , and ~.
• Special characters and quoting: Certain 'special' characters (e.g., parentheses, angle brackets, square brackets, colon, semicolon, at sign, backslash, comma, period, double quote) are not allowed in 'atext' outside of a 'double-quoted string'. Inside a double-quoted string, most characters are allowed, but some must be escaped with a backslash.
• Local part complexity: The local part can be a sequence of one or more atoms separated by periods, or it can be a single double-quoted string. There are specific rules regarding leading, trailing, or doubled periods that must be observed for atomic local parts.

Key considerations

• Referencing the RFC: For precise and authoritative information on email address syntax, always refer directly to the RFC 5322 document, particularly section 3.2.3 on lexical tokens and 3.4.1 on address specification. This is key for resolving RFC compliance errors.
• Practical application: While RFC 5322 defines the theoretical limits, practical email implementations and common coding patterns often adopt a more conservative subset of allowed characters for simplicity and broader compatibility across various email systems.
• Quoted strings usage: Although 'double-quoted strings' offer significant flexibility, they are less commonly seen in everyday email addresses. When they are encountered, ensure your parsing and validation logic correctly handles escaped characters and the specific rules for such strings.
• Domain part: The domain part of an email address must conform to DNS naming conventions, which are generally more restrictive than the local part. This means it primarily allows alphanumeric characters and hyphens, but not at the beginning or end of a domain segment. This aligns with rules for hyphens and dashes.