I do understand the point you're making; that sometimes there is a "good enough" solution, especially where a full RFC implementation seems like overkill in the context of a more closed environment.

But you mentioned that you've seen what happens when two MTA's can't agree on what is and is not valid. I imagine that can be quite a problem. So why make things worse by supporting the building of solutions that fail to meet the RFC? If a particular network is going to send and receive email over the internet, or if a webpage is going to be validating email addresses that are to be valid on the internet, a mostly-correct solution is going to be somewhat-incorrect. I know that CGI Programming with Perl suggests a mostly-correct solution for matching email addresses. But there exists (in the CPAN modules) solutions that are fully correct. If writing ones' own fully-compliant solution is too much work, there's always those modules.

The OP suggested that he couldn't use modules because this ultimately will be ported to another language. ...well at least learn from the modules out there how to go about the task. Or invoke Perl from that other language's code to perform the test. ...or ask the question in a PHP-oriented group, where undoubtedly it has been answered before. ;)