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ListMessenger exceeds expectations as a well designed, easy to use, lightweight mailing list management solution for your website. ListMessenger is feature packed but because of its intuitive design remains simple to use and easy to maintain.
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ListMessenger is giving me an error about not being able to find a file.

If you are getting a PHP error stating that ListMessenger cannot find a specific file, the first thing I would do is try re-uploading the files from the distribution file to your web server. Chances are that the file either was never uploaded (maybe missed by your FTP client) or you have accidentally deleted it.

ListMessenger can not accidentally delete one of its own files and trust me, it's not something that magically would disappear (accept if you're a Windows user, then just be quiet and accept it. hehe).