Having conversed via comments, it seemed that a slightly more aggressive exploit protection was required and so the plugin now has 2 strong mechanisms that also cross check with each other before a mail is created. I hope that this will help completely stop any exploits but of course, the spammers are getting wise so please let me know if you have any ideas or evidence of vulnerabilities. As I cannot easily check the exploits on the code myself, your information is valuable to all of the other users! The comments on the plugin page have been archived as the list was getting very long but there is a link for anyone that wishes to read through them. Please read the instructions carefully as things have changed and it should run out of the box as usual! PXSmail page can be found here.

Update:
If you are running version 1.3 please ensure that you get the latest update (1.4) to fix a bug that was preventing emails from being sent out.