Hi Marin,
Thank you very much for your reply and help, I really appreciate it.
I tested my example link (<a onclick="href='mailto:'+'first.name'+'@'+'domain.com'+'?subject=hello world'" href="#">Email Me.</a>) in all versions of IE, using your attached editable iframe. It works and does not show any inconsistencies like my Sitecore environment is. Very odd but a good thing I guess, as this means it can be fixed and that the browser isn't 100% overriding it.
I haven't tried the other points you mentioned but now knowing my above response, is there anything else you recommend?
Either way I'll raise a ticket with Sitecore as they might be able to resolve this issue also :)
Kind regards.