sending data via ajax https problem

Greetings.

I have a mailing list form that allows people to submit their email to be on an email list. It works great on https://mymaindomain/subblog

However if I try to use http://www.subblogmappeddomain.com it fails because it is trying to submit the data via

https://www..subblogmappeddomain.com/wp-admin/admin-ajax.php

which does not exist.

looking at the plugins code it looks like it is generating that url to send data on via:

var ajaxurl = '<?php echo admin_url('admin-ajax.php'); ?>';

Wondering if:

A. This is an issue with the domain mapping plugin

or

B. should some how I be sending the form via:
https://mymaindomain/subblog/wp-admin/admin-ajax.php

and if the answer how could I modify the code to achieve that?

Thanks.

-Ben