A valid email is required
Executive summary
A valid email address must follow the standard local-part@domain structure defined in Internet standards and used by most services; practical checks include syntactic rules (one @, allowed characters, segment lengths) and delivery checks (domain/MX existence or a verification email) to ensure messages actually reach a recipient [1] [2] [3].
1. What “valid” means in technical terms
At the protocol level an email is “valid” if it conforms to the local-part@domain syntax established in RFC-derived guidance: a local-part (username) left of the @ and a domain to the right, with the local-part allowed up to 64 octets and the domain up to 255 octets in practice — those are the formal size ceilings many validators use [1] [4].
2. The character and label rules most validators apply
Common validation rules used by services require letters and digits and permit some special characters in the local-part (periods, underscores, dashes and a limited set of other characters), disallow consecutive or trailing punctuation in many implementations, and require domain labels (the dot-separated parts) to be no longer than 63 characters and not begin or end with a hyphen [5] [6] [7].
3. Practical checks beyond raw syntax
Because RFCs permit more exotic forms than many platforms accept, production validation usually combines syntax checks (exactly one @, length limits, allowed characters) with domain checks — confirming the domain has a valid top-level domain (TLD) and that DNS/MX records point to mail servers — and often ends with a verification email (double opt‑in) to confirm deliverability [2] [3] [1].
4. Why some seemingly legal addresses are rejected in the wild
Many mail providers and web forms intentionally restrict the legal set from RFCs: they reject characters like + or uncommon punctuation, enforce their own length limits, or block certain domain patterns for UX or security reasons; implementers do this even though the protocol technically allows wider variations, so a string that is RFC‑legal can still be refused by a service [1] [8].
5. Tools, vendors, and hidden incentives
A crowded market of email-verification vendors promises real-time syntax checks, MX pings and deliverability scoring — vendors like Clearout loudly claim high accuracy and business ROI, and marketing language appears across guides that advise using paid verification APIs to “fix” invalid addresses; readers should note the commercial incentive in vendor content when weighing recommendations [6] [9] [10].
6. Implementation best practices to meet “A valid email is required”
To enforce “a valid email is required” in a form, combine client-side format hints (HTML5/email input patterns) with server-side validation against the local-part@domain rules, apply domain label length and hyphen rules, optionally do an MX lookup to confirm mail servers exist, and where account ownership matters, complete the workflow with a verification message [3] [2] [9]. If a candidate address passes syntax but the domain has no MX records or the verification link bounces, treat it as invalid for practical purposes [3] [9].
7. Limits of available reporting and remaining uncertainties
The cited guides and vendor posts describe the common validation steps and protocol ceilings, but they vary in detail and sometimes promote commercial services; the sources do not provide a single definitive regex or universal blacklist that all systems use, so any implementation must pick a pragmatic policy that balances RFC permissiveness, user experience and deliverability needs [1] [6] [8].