A new mail filtering rule that is applied when the folder is opened will trigger both when the new mail folder is opened and when the new mail folder is open when the message arrives.  What I suspect is happening in your case is that the user is using Preview mode and the new mail folder was not the active folder when Pegasus Mail is minimized.  In that case, the new mail filter will not trigger until the new mail folder is activated.  The solution is for the new mail folder to be open at all times.

Be aware that a forward filter like you describe will run each time the new mail folder is opened and will trigger on every unread message that meets the filter criteria.  The result could be the same message being forwarded multiple times unless it is marked as read or moved out of the new mail folder after the first forward.

I am evaluating PegMail (latest win 64 build) and have discovered that a new mail rule (in this case to forward the msg) does not work when PegMail is minimized to either the Windows taskbar or the  Windows notification icon area.

The rule will only execute upon changing the PegMail window to a visible state.

So if I have PegMail minimized and receive an email that meets the forwarding criteria, it does not forward until a user clicks to show the forwarding instance of PegMail on the desktop.

It works fine when the PegMail UI is already in a visible state, and no user intervention is required in this case.

Is there a setting or commandline parm that could allow it to forward when minimized? 

Tested on Server 2012 R2