Inconsistent execution, producing broken HTML and prompting to install plugin

I have installed GTSpam on Drupal 5. Using FCKEditor.

All roles are set to use the Full HTML input format.

I have disabled source formatting and output formatting in the FCKEditor profile that's in use by all roles.

No other filters are active for Full HTML. As far as I can tell, I've completely isolated GTSpam from any other filters, at least as much as it is in my power to do so.

Here's are a few typical behaviors -- code in this case is copied out of Firefox; the Safari (3) results are more or less identical with regard to (broken) rendered output, but differ with regard to the underlying code.

If I type the email link like this:
<a href="mailto:escoles@brandcoolmarketing.com">email me</a>
I get this out the other end:
<a href="mailto:%3Ca%20href=" modules="" gtspam="" meiltu.php?a="JkZbHAoWFyZaBlU7AUMIIBYJBwEPAwUmQChZOQkD&quot;" style="white-space: nowrap;" target="mfrescoles" title="Send email">escoles<img src="/modules/gtspam/monkey.gif" alt="@" style="border: 0pt none ;">brandcoolmarketing.com</a><object type="text/html" style="width: 0pt; height: 0pt;" name="mfrescoles"></object>"&gt;email me
Which renders as this:
escoles@brandcoolmarketing.com">email me

If I type the email link like this:
<a href="mailto:escoles@brandcoolmarketing.com">escoles@brandcoolmarketing.com</a>
I get this out the other end:
<a href="mailto:%3Ca%20href=" modules="" gtspam="" meiltu.php?a="JkZbHAoWFyZaBlU7AUMIIBYJBwEPAwUmQChZOQkD&quot;" style="white-space: nowrap;" target="mfrescoles" title="Send email">escoles<img src="/modules/gtspam/monkey.gif" alt="@" style="border: 0pt none ;">brandcoolmarketing.com</a><object type="text/html" style="width: 0pt; height: 0pt;" name="mfrescoles"></object>"&gt;<a href="/modules/gtspam/meiltu.php?a=JkZbHAoWFyZaBlU7AUMIIBYJBwEPAwUmQChZOQkD" style="white-space: nowrap;" target="mfrescoles" title="Send email">escoles<img src="/modules/gtspam/monkey.gif" alt="@" style="border: 0pt none ;">brandcoolmarketing.com</a><object type="text/html" style="width: 0pt; height: 0pt;" name="mfrescoles"></object>
Which renders as this:
escoles@brandcoolmarketing.com">escoles@brandcoolmarketing.com

If I type the email link like this:
<a href="mailto:escoles@brandcoolmarketing.com?subject=message%20subject&amp;body=message%20body">escoles@brandcoolmarketing.com</a>
I get this out the other end:
<a href="mailto:%3Ca%20href=" modules="" gtspam="" meiltu.php?a="JkZbHAoWFyZaBlU7AUMIIBYJBwEPAwUmQChZOQkD&quot;" style="white-space: nowrap;" target="mfrescoles" title="Send email">escoles<img src="/modules/gtspam/monkey.gif" alt="@" style="border: 0pt none ;">brandcoolmarketing.com</a><object type="text/html" style="width: 0pt; height: 0pt;" name="mfrescoles"></object>?subject=message%20subject&amp;body=message%20body"&gt;<a href="/modules/gtspam/meiltu.php?a=JkZbHAoWFyZaBlU7AUMIIBYJBwEPAwUmQChZOQkD" style="white-space: nowrap;" target="mfrescoles" title="Send email">escoles<img src="/modules/gtspam/monkey.gif" alt="@" style="border: 0pt none ;">brandcoolmarketing.com</a><object type="text/html" style="width: 0pt; height: 0pt;" name="mfrescoles"></object>
Which renders as this:
escoles@brandcoolmarketing.com?subject=message%20subject&body=message%20body">escoles@brandcoolmarketing.com

Also getting a notice (in Firefox 3) that "Additional plugins are required to display all the media on this page."