the solution pack compiles the email then passes it to the SMTP server.
it is up to the SMTP server to then look at each address it needs to send the message to, then relay the email.
Since it is sending to some addresses, it is being compiled and passed to the SMTP server.
It is the SMTP server that is failing to perform the relay for that particular address.
you need to work with the host to determine why the SMTP server is not preforming the relay for that address.