Email Standards Project : The Email Standards Project is about working with email client developers and the design community to improve web standards support and accessibility ...
regex101 : Free PCRE-based regular expression debugger with real time explanation, error detection and highlighting.
Guide to CSS support in email clients : Designing an HTML email that renders consistently across the major email clients can be very time consuming. We've put together this guide to save you...
This page compares regular expressions that validate e-mail addresses in order to find the best one.