Question
Why the Email Validation API shows an 'Invalid' verdict for perfectly good email addresses?
Product
Twilio SendGrid (Email)
Answer
In most cases, the reason is the email address itself, particularly if it contains two or more consecutive special characters, such as dots or underscores or a combination of them. In these cases, the check 'has_valid_address_syntax' is marked as 'False' and therefore, all the other checks will auto-fail.
Please note that addresses with this syntax are very uncommon which is why these 'Invalid' verdicts are considered Edge cases, and therefore, not accounted for within the design of the Email Validation API.
Additional Information
Although there is no rule in the RFC5322 standard that prevent such email address from being created, the Email Validation tool has been designed keeping in mind compatibility across different email services. Gmail, for example, doesn't accept the underscore in their email addresses, although the RFC5322 clearly states that _ is a valid character within an email address.