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

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.

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.

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.

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.