By default, the list server will not distribute messages that it detects as being automatically generated (i.e. by a program).

Now, normally this is a good thing, as it avoids the potential for mail loops, but obviously there are cases where you as list owner would wish to allow this - examples include where you are using a list to aggregate (and distribute to number of recipients) automated mail such as alert emails from IT systems, or automatic notifications from applications (e.g. Calendar notifcations)

You can do this by using the web interface. 

Log in as normal, then navigate to the list you wish to modify. 

Select Admin -> Edit List Config -> Sending/Receving Setup

On the right hand part of the screen, there should be an option : 

Setting this to 'disabled' will allow your list to receive messages from automatic processes.