Yes. Sorry to say, but this was a huge red flag for me - if the author is apparently unaware of a project that it is that well-known, he has no business writing mail software. (I know that he comments on this very page that he is aware of GNU Mailman...)
check the comments on his blog. he knew about it ahead of time and didnt think it was an issue because someone could just google 'ruby mailman'. apparently the idea of encroaching on another project's name never occurred to him as an extremely impolite ( to put it kindly ) thing to do.
