Hey Seamus,
Support has always higher priority than the new version - at the end you need to have a working site, right...
so here is what has changed for 2.5. Go to components/com_hotspots/controllers/hotspot.php line 245
JUTility::sendMail($mailer->From, $mailer->FromName, $mail, $subject, $message, true, $mailer->cc, $mailer->bcc, $mailer->attachment, $mailer->ReplyTo, $mailer->FromName);
and change it to:
JUTility::sendMail($mailer->From, $mailer->FromName, $mail, $subject, $message, true);
After that enable again the Mail function and test if you are able to properly submit a Hotspot.
Cheers,
Daniel