Hi Andy,
According to your description, the issue can be caused by the firewall setting of PTR record checking with the destination email server.
I have checked the DNS setting of your domain, every DNS recorders are fine.
Under normal conditions, the server will check the SPF/TXT record for filter the Spam Mail and you have finished setup this recorder, but some Exchange server will check the PRT records for filter the Spam Mail, that the root cause of this issue.
Since that Exchange server is managed by their IT team, so I suggest you contact their IT team for add your domain into the white list.
Thanks,
Wilfred Ying