Comment 1 for bug 266318

This looks like Mailman 2.1.5 or earlier. This problem was
fixed in 2.1.6. At line 210 in your
Mailman/Handlers/ToDigest.py you will see

    while msg is not None:
        if msg == '':
            # It was an unparseable message
            msg = mbox.next()
        msgcount += 1

You need to add a continue so this becomes

    while msg is not None:
        if msg == '':
            # It was an unparseable message
            msg = mbox.next()
            continue
        msgcount += 1

In the mean time, if you don't mind having your digest out
of sequence, just move the digest.mbox aside. Then you can
patch ToDigest.py and straighten out the digest situation.
Note that you probably don't want to both run bin/unshunt
and replace the digest.mbox as that will result in
duplicates, but if messages haven't reached the list, you
need to run bin/unshunt to finish processing them.