Hey, I really like your plugin - but I experienced some problems with it.
First, the template throws an error for me. Only when I delete all "{$email->language->addToStack.. }" phrases it does not crash anymore. I would like to fix it by myself but I am not that familiar with IP plugins.
Example error from nginx log: 2016/01/27 20:10:51 [error] 8217#0: *684264 FastCGI sent in stderr: "PHP message: PHP Fatal error: Call to a member function addToStack() on a non-object in +removed+/system/Email/Email.php(439) : eval()'d code on line 6" while reading response header from upstream
Second, it does not work when SMTP is enabled. When SMTP is enabled with following details: Sender E-Mail: service@example.com, SMTP Login: username, SMTP Password: password and I send an invitation with a user whose email is user@otherexample.com he tries to send a mail with user@otherexample.com per SMTP with the login details of the support mail, which is definitely not supposed.
If you need any further information, it's no problem, just ask. I'd like to see a fix as fast as possible