I tried libspf and found out that it has a bad code quality. It contains code like

|      if (rr_data == NULL)
|      {
|        rr_data = xmalloc(substr_len + 2);
|      }
|      else
|      {
|        rr_data = xrealloc(rr_data, (i + substr_len + 2));
|      }
|      strncat(rr_data, (char *)rd_ptr, substr_len);

at several places and that is really nothing which should touch my or somebody
else's mails.  There were memleaks also, but I can not remember the details and
can not find the libspf forum anymore where I posted my bugreports.

So I will not fix that, unless:

* somebody else packages libspf and proofs that he fixed all issues
* somebody else packages an alternative spf implementation

This seems to be a good reason to not to use SPF. Thank you for pointing out.

The second reason is that a lot of spam went through milter here because
spamming relays are using MailFrom from random domains with globally enabled SPF
records. So the spams are going through milter-graylist with no problem...