Create dynamic external link URL without filtering or stripping protocol

Question

In my module I'm creating a dynamic external link to launch a local FileMaker database client and run a FileMaker script that finds a record like so:

FMP://192.168.123.123/DatabaseName.fmp12?script=FindRecord&RecordID=12345

The problem is no matter whether I use the Drupal 7 link l() function with attribute array absolute = true or external = true or if I assemble the link markup myself, either the anchor's href attribute gets stripped entirely or the protocol of FMP gets replaced with HTTP.

What do I need to do to let Drupal know this URL is safe to be outputted unmolested in outgoing emails?

Answer 1

I realised it's not Drupal changing the URL but GMail that doesn't like the protocol.

So I created a simple PHP redirect script and link from my Drupal module to that instead e.g.

<?php

header("Location: FMP://192.168.123.123/DatabaseName.fmp12?" . $_SERVER['QUERY_STRING']);
exit;

?>