Postfix SPF softfail

[xy16644]

When I installed Postfix I enabled the SPF option. Today I setup SPF in my main.cf file as follows:

smtpd_sender_restrictions = reject_spf_invalid_sender
                                          permit_spf_valid_sender

smtpd_recipient_restrictions = reject_spf_invalid_sender
                                             permit_spf_valid_sender

spf_received_header = yes
spf_explanation = "%{h} [%{i}] is not allowed to send mail for %{s}"
spf_reject_code = 550
spf_mark_only = no

When I receive an email from a domain that has an SPF record the header says:

Received-SPF: pass

But when I send a fake email to test that Postfix/SPF will reject the email I get:

Received-SPF: softfail (mail.domain.com: transitioning domain of gmail.com does not designate xxx.xxx.xxx.xxx as permitted sender) client-ip=xxx.xxx.xxx.xx; envelope-from=bob@gmail.com; helo=mail05.domain.org

So how do I tell Postfix/SPF to reject all email that fails an SPF check?