when the email is received by some addresses but not others, there are 2 possibilities:
1) The email is being trapped as spam by the clients spam filter.
Have them check there spam filters to see if the email is being regarded as spam.
2) The SMTP server is refusing to send the message to those addresses for some reason, you will need to contact the host to determine why the SMTP sever is not relaying the email message to those addresses.