just got alerted to your post so i ran a few quick checks.
Sending into Post Office from an old .fsnet (Wanadoo/Orange) address, using non-Orange, authenticated smtp, seems to work ok.
It arrived within seconds.
It's true that incoming and outgoing Post Office mail does use Westcoastcloud/Netintelligence.com (same company).
Mail is routed through their servers for checking and they add a footer to the mail, telling you that.
also the headers show the routing to them, carrying several lines, such as:
Received: from mailfilter.iomart.com (glamf04.netintelligence.com [18.104.22.168]
The footer seems to randomly use either name, Westcoastcloud or Netintelligence, but the headers all show netintelligence.com
So it is certainly possible that they could be blocking some mail, if they find cause to suspect it.
But seems to be working ok for me.
I also tried sending from Post Office, Hotmail, AOL, Yahoo and Gmail and they all arrived, again within seconds,
So, maybe your friend has email scanning as part of their AV suite?
But are your emails being returned to you with any error code?
If so, is your Orange smtp authenticated? that might help,
Are you using port 25? Does your Orange smtp work with port 587?
but just let me know if you want me to try something else