where it works sending to some email addresses, but not sending to others, it is is a problem on the SMTP server, the sSMTP server is refusing to relay the email for that address for some reason. you would need to access the SMTP server error logs to find why the message is not being relayed for that address.
you want to try using a different SMTP server.
if you have a Gmail account, and your host supports pear mail, you can send through the Gmail SMTP server.
set the from address to your GMail address, in the server settings, set the mail object to pear mail for SMTP, enable authentication, use your Gmail address for the username and your Gmail password for the password.
for the SMTP server, enter:
and for the port enter: