When using a mailto link chances are that it doesn\'t do anything for the user if he doesn\'t have an email client setup, or didn\'t setup his webmail to be his default client (
You cannot detect whether or not a user has an email client setup. So I would suggest you to setup a server-side form for the user to contact you.
Beneath or above this you can provide the user a link explaining him that if he wants to contact you directly through his main e-mail account, he can click this link (this link being a mailto: link).
By providing him two ways of contacting you (webform or e-mail client), you give the user the opportunity to choose which one he wants to use. So it's up to him to realize whether or not he has an e-mail client installed and whether or not he wants to use it.