Hello,
I was hoping to continue to use your plugin for my website, but when I activate it, I get these errors:
Notice: Undefined index: act in C:\xampp\xampplite\htdocs\wordpress\wp-content\plugins\newsletter\plugin.php on line 652
Notice: Undefined index: page in C:\xampp\xampplite\htdocs\wordpress\wp-content\plugins\newsletter\plugin.php on line 68
Not sure what the issue is, but it also throws some errors for wordpress' own files (wp-includes). Perhaps you could check this out?