Reference

An excellent deep-dive into the workings of email: The site includes a ‘Tools’ section which includes a large number of email-related tools:

The SMTP Field Manual is a website that houses a “collection of raw SMTP error code responses from major email service providers and spam filter services”. It provides a cross-listing between various email service providers – such as Apple, Google, Outlook, Yahoo, Yandex, and more – and various SMTP error codes that they may be…

From an article titled “Your E-Mail Validation Logic is Wrong“ dispelling our notions about the ease of email validation