This error is not a problem with your email message or anything associated with your organization. Not being able to connect successfully to the MX record is a problem with the DNS record, which is controlled by the email provider.

Troubleshooting MX record connections is out of the scope of support. For more information on understanding MX records, see the link below and type in the domain for the bounced email such as(email@domain.com) and you will see the MX record is not resolving.

http://www.mailwise.com/spt_tools.htm
Whitelist the following IP address ranges on the email server:

216.235.196.0/22
216.235.200.0/21