looking at 
line 90:
def test_mail(mail, spamc=False, cmd=False):
    """Test a email with spamassassin."""
    score = "0/0\n"
    orig_code = None
    spamassassin_result = None
    returncode = None

    if cmd:
        satest = cmd
    elif spamc:
        # let spamc process mails larger than 500 KB if ISBG forwards them
        satest = ["spamc", "-E", "--max-size=268435450"]
        satest = ["spamassassin", "--exit-code"]

    proc = utils.popen(satest)

The limit 268435450 seems to be hardcoded.
But 268.435.450 bytes is 268.435kBytes is 268MBytes... So. Why is my 5MB Mail not scanned?

Anyway, should 
satest = ["spamc", "-E", "--max-size=268435450"]

not be something like
satest = ["spamc", "-E", "--max-size="+sa.maxsize]
(note the object sa is not available here I think).

Now, for some reason, I now do not get the error
['spamc', '-E', '--max-size=268435450'] error for mail 151

anymore, but instead:

['spamassassin', '--exit-code'] error for mail 151
NoneType: None

I have in between deleted the .cache/isbg/track* files