Did you mean mbox or .mbox?

Following from mailx man page:
Incoming mail is stored in a standard file for each user,
called the mailbox for that user. When the mail utilities
are called to read messages, the mailbox is the default
place to find them. As messages are read, they are marked
to be moved to a secondary file for storage, unless specific
action is taken, so that the messages need not be seen
again. This secondary file is called the mbox and is nor-
mally located in the user's HOME directory (see MBOX in
ENVIRONMENT for a description of this file). Messages can
be saved in other secondary files named by the user. Mes-
sages remain in a secondary file until forcibly removed.

( or did you mean mail.rc ?)
After executing the system startup file, the mail utilities
execute the optional personal startup file $HOME/.mailrc,
wherein the user can override the values of the internal
variables as set by the system startup file.