since its an upcoming feature in the alpha version (5.7.0-B2366) I couldn't find an explanation in the current documenation. When I do the diagnostic with a selected domain it gives me this error:
Test local connect
Connecting to TCP/IP address in MX records for local domain domain somename.domain....
Trying to connect to host mail.somename.domain...
Trying to connect to TCP/IP address XXX.XXX.XXX.XXX on port 25.
ERROR: It was not possible to connect. Error: Could not connect to XXX.XXX.XXX.XXX on port 25 since this would mean connecting to myself..
ERROR: Failed to connect to all servers.
What's intended with this feature? Because it's called "test local connect" and when the domain is local it gives the error?