Backslash Automatically Added Before Apostrophe In Subject Field After Sending

Hello again,

A user on our site with the Email Users plugin would be able to send the same message more than once to different users and would like to know why is it that after a message is sent, there is a slash \ in before an apostrophe, and if there is a fix for that. A new backslash is added every time the send button is pressed.

Thanks.