purgembox.py

#! /usr/bin/env python
#
# Goes through an mbox (filename given in argv[1]) and
# purges messages older than thirty days.
# (and newer than year 3000 to pick up corrupt headers)
#
import sys
import platform
try:
    pv = platform.python_version_tuple()
    if(pv[0] < 2 or pv[1] < 6):
        raise ImportError
    import mailbox
    import email.utils
    import time,datetime
except ImportError:
    print "Incompatible version of Python: %s" % platform.python_version()
    sys.exit(1)

if(len(sys.argv) < 2):
    print "Usage: %s mboxfile" % sys.argv[0]
    sys.exit(1)
mb = mailbox.mbox(sys.argv[1])
tokeep=0
tolose=0
errors=0

def deletemessage(key):
    mb.remove(key)

if(mb):
    try:
        mb.lock()
        keepdate = datetime.datetime.now()-datetime.timedelta(days=30)
        print "Keeping everything since %s" % keepdate
        time.sleep(5)
        for messagek in mb.keys():
            message = mb.get(messagek)
            try:
                emdate = datetime.datetime.fromtimestamp(time.mktime(email.utils.parsedate(message['Date'])))
                # keepdate = datetime.datetime(2010,8,29)
                if(emdate < keepdate or emdate > datetime.datetime(3000,1,1)):
                    print "emdate is to be LOST: %s" % emdate
                    tolose+=1
                    deletemessage(messagek)
                else:
                    print "emdate is to be KEPT: %s" % emdate
                    tokeep+=1
            except TypeError:
                print "error on messagedate: %s" % message['Date']
                errors+=1
                deletemessage(messagek)
            except ValueError:
                print "error on messagedate: %s" % message['Date']
                errors+=1
                deletemessage(messagek)
    finally:
        mb.flush()
        mb.unlock()
print "Going to keep %d lose %d" % (tokeep,tolose)

No parameter support on this, it's all just in the file but you can use this to delete a range of messages in an mbox file.

See:
http://docs.python.org/library/mailbox.html