<form method="post" action="./mailgate.pl" enctype="multipart/form-data" name="msgform" onSubmit="SwitchDeleteButton('DeleteButton', 'DeleteButtonNone')">
...
<a name="DeleteButton"><input class="button"  type="submit" name="submit" value="Delete"></input>
</a>
<a name="DeleteButtonNone" style="display: none"><input class="button" disabled type="submit" name="none_submit" value="Delete"></input>

function SwitchDeleteButton(spanname1, spanname2)
{
	var button = "<input disabled class=\"button\" type=\"submit\" value=\"Delete\">";

	var elements1 = document.getElementsByName(spanname1)
	for (var i = 0; i < elements1.length; i++) elements1[i].style.display = 'none'

	var elements2 = document.getElementsByName(spanname2)
	for (var i = 0; i < elements2.length; i++) elements2[i].style.display = ''
}