some host will not relay the email address if the from address is not a valid address for your domain.
set the from address to a valid address from your domain.
you can set the reply to address to use the dynamic value from your form.
in most cases, leaving the smtp server blank is ok but you will need to check with your host if it is not working to determine the correct smtp server to use for sending an email through the php mail() function.
if it is all configured correctly, and the email is still not being sent, it is a problem at the SMTP server end, for some reason the SMTP server is not relaying the email to the recipient. to determine why, you will need to contact the host to have them look into the smtp servers error logs.
