I would check it first, make sure it can get to the internet, that it's forwarders can or something in the chain can resolve DNS.
It's a Windows server yes, so it's likely you've got windows DNS server turned on, which means the interface will be set to localhost for active directory resolution etc, in that case, check the DNS servers forwarding config, run the tests, make sure it can talk to the internet.
Or just turn off DNS checks for email in osTicket..
Admin panel -> Emails -> Settings -> Verify Email Address
If that fixes it, then you know it's a server DNS issue.