Network check failed -- IP address 'xxx.xxx.xxx.xxx' associated with host name 'xxx.example.com' in DNS cannot be confirmed as this host's IP address.
Judging by this response I assume that you are required to have rdns on the host name that you use.
I do realise that according to the RFC you are meant to have RDNS on your mail server, however in most cases when interfacing with other mail servers, its not a requirement.
I therefore assume that the only solution to this problem is to use a machine that has an IP address with RDNS assigned or get RDNS assigned to that IP.