You need to update your DNS records. Go to /etc/postfix/main.cf and look for this line and edit it
## change this with your actual mailserver domain
myhostname = mail.myprimarydomain.com
# also disable ipv6 andwhich in some cases fixes the bounce
# You must stop/start Postfix after changing this parameter
# inet_protocols = all
inet_protocols = ipv4
Likelihood its just ipv6 issue. Learn more - Postfix IPv6 Support