Server IP Address points to the wrong domain name

After setting up ajenti, the ip points to the wrong domain name. Typing in the IP should go to MY domain (webnetvn.com) however it goes to one of my cleints domain instead (designsma.com) which is really messing up the mail system. anyone sending mail to a yahoo email address recieves the error

450 service permits 2 unverifyable sending IPs - webnetvn.com is not 104.236.18.21 (designsma.com)

All of my clients domains are set so their MX record points to mail.webnetvn.com. how do i fix this so that the panel thinks the default domain for everything is webnetvn.com and not designsma.com?