The Outbox or Remote domain host-name is set in either the
Outbox/Connection or the
Remote Domain/Connection page.
Your host-name should be the name that will be resolved by a reverse lookup for your ip address, or at the very least a valid name used in a DNS which resolves to your IP.
For example :
lookup mx0.ftgate.com = 188.8.131.52
lookup 184.108.40.206 = 225.128-255.16.224.195.in-addr.arpa = mx0.ftgate.com
So it resolves to the same address both ways.
This is the correct way to have a host defined when sending out using MX records.
If both paths are not there then some servers will reject.
To configure this in your DNS server you will have to contact your ISP/DNS hosting company.
Some servers will not allow the host-name to be set to the IP address. In this case, if you have no valid reverse lookup host-name, you should use your domain name.