EmailString

EmailString is a term used in software development to describe a string data type that stores an email address. It appears in user profiles, contact forms, authentication flows, and mailing lists as the primary representation of a recipient or contact.

In code, emailString is typically treated as a single address, though some interfaces allow multiple addresses in a single string, separated by commas. Validation often starts with syntactic checks (such as the presence of an at-sign and a valid domain) and may progress to deliverability tests or SMTP verification in specialized systems.

Formally, an email address consists of a local part, an '@' symbol, and a domain part. The domain is generally case-insensitive; the local part can be case-sensitive depending on the mail server. Many applications adopt a pragmatic approach and convert inputs to lowercase to normalize comparisons, though this may not preserve all original semantics.

Length limits are defined by standards: the maximum total length is commonly 254 characters, with the local part up to 64 and the domain up to 255. Real-world storage often uses database fields sized to accommodate these limits, and some systems permit internationalized addresses using UTF-8 for the local part and IDNA for the domain.

Security considerations include input sanitization to prevent header injection, avoiding exposure of emails in logs or error messages, and validating before use in mail headers or deliveries. Internationalized email (SMTPUTF8 and IDNA) broadens support but may require additional handling and compatibility checks.