When the email sends to some domains, but not others, it points to a setting on the SMTP server that is preventing it from relaying emails to that domain.
This can also happen if the From and To address are the same. Double check that the email address you are trying to send the email to is not the same as the email address being used for the From Address. If these two are different, then contact the host to see if they can look into the smtp servers error logs to see why it is refusing to relay.
