Mutt - Procmail Question

[Jijo Jose A <jijojose@sancharnet.in>]

hi all
still i used procmail 3.21.20010831.3.22pre-1 for mail processing and mutt
1.3.22-1 as MUA, to remove the unwanted headers i wrote '|cat | formail -k -X From:
-X Return-Path: -X Date:...... >>my_mbox' in .procmailrc as the action line 
for a condition. but i started 'mutt -f my_mbox ', my_mbox is not a mailbox....
error will occured. 
how can i solve this ?
TIA
-jijo jose