When multiple mail boxes are enabled, they are titled mail1.box, mail2.box, and so on and the router uses these mail boxes for routing messages.
To force the router to use “mail.box”, add the parameter to the notes.ini file:

Mail_Enable_Mailbox_Compatibility=1

The parameter keeps the “mail.box” file and still will create mail2.box, mail3.box, and so on.
This is useful if you have a third party applications that requires the existence of a “mail.box”.