It's impossible to tell what could be wrong with so little information.
Are you positive you are using the correct smtp server for your host?
What have you used for the from address? make sure that it is a valid address for your domain.
In some cases, entering a from display name can cause problems, try leaving that blank.
In the SMTP server settings section, there is a checkbox for "Include From address as an Extra Parameter" try checking that.
are you getting the from or to address from the form?