You want the string that the user will enter to be sent straight to your client's E-mail inbox? Don't you think that might be asking for trouble? Someone could really enter anything they want, a million times over, and flood your poor client's inbox. Not to mention the sorts of profanity and general obcenities that could meander on through.
My suggestion to you: take a look through the PHP manual. Study how to pick apart a string (to make sure the e-mail address is formated correctly [name@domain.xxx]), study how to take this string and store it somewhere, like in a secured file on your server (hint: check out fileSystem functions and the fopen(), fwrite(), and fclose() functions).
This won't take care of every kind of attack, but it'll make sure the user doesn't use spaces and extended or illegal characters, and that they at least have an @ sign and a . in the string. Plus, the file can be accessed at the convenience of your client, instead of having hundreds of e-mails flooding his personal e-mail address.
let me know how it turns out, and what you decided to do.