Email Validation behaviour

[JeremySkinner]

A common bug report that we receive is that that the email validator doesn't work correctly, and identifies something as a valid email address when it shouldn't.

This is not a bug and is the correct/intentional behaviour.

The email validator is intentionally naive, and uses the same logic as ASP.NET Core's default [EmailAddress] attribute for consistency, which is just to perform a simple check that a string contains an "@" character. This is explained in our email validator documentation here: https://docs.fluentvalidation.net/en/latest/built-in-validators.html#email-validator

It is also mentioned in the 9.x upgrade guide, when we switched from a regex to using the simple check:
https://docs.fluentvalidation.net/en/latest/upgrading-to-9.html#default-email-validation-mode-changed

This discussion may also be of interest as to why trying to build comprehensive email validation regex is the wrong approach: https://www.reddit.com/r/csharp/comments/hlzpwn/why_is_emailaddress_attribute_validating_as_true/ (TLDR; the best way to validate email addresses by sending a verification email to the address. At the UI-level just perform a simple/basic check).