extract_from.py

import mailbox

def extract_from(mbox_file):
    mbox = mailbox.mbox(mbox_file)
    
    for message in mbox:
        print(message['from'])

if __name__ == "__main__":
    import sys
    if len(sys.argv) != 2:
        print(f"Usage: {sys.argv[0]} MBOX_FILE")
    else:
        extract_from(sys.argv[1])

# use like python extract_from.py path_to_your_mbox_file