Why doesn't SPF work?

Question

SPF

Set up a TXT record in DNS:
@ "v=spf1 +a include:_spf.google.com -all"
Postfix redirects from [email protected] to [email protected] Email
goes to spam:

google.com: domain of transitioning [email protected] does not designate IPOFMYSERVER as permitted sender)

What am I doing wrong? Or should I wait 72 hours?
dig TXT mydomain.com
and viewdns.info/dnsrecord/?domain=mydomain.com
show: "v=spf1 +a include:_spf.google.com -all"

spoiler

Delivered-To: [email protected]
Received: by 10.64.166.8 with SMTP id zc8csp729648ieb;
Fri, 29 Apr 2016 04:51:01 -0700 (PDT)
X-Received: by 10.194.230.69 with SMTP id sw5mr21150135wjc.144.1461930661579;
Fri, 29 Apr 2016 04:51:01 -0700 (PDT)
Return-Path:
Received: from mydomain.com ([46.101.247.118])
by mx.google.com with ESMTP id m14si16984275wjw.192.2016.04.29.04.51.01
for ;
Fri, 29 Apr 2016 04:51:01 -0700 (PDT)
Received-SPF: softfail (google.com: domain of transitioning [email protected] does not designate 46.101.247.118 as permitted sender) client-ip=46.101.247.118;
Authentication-Results: mx.google.com;
spf=softfail (google.com: domain of transitioning [email protected] does not designate 46.101.247.118 as permitted sender) [email protected]
Received: from mail-lf0-f50.google.com (mail-lf0-f50.google.com [209.85.215.50])
by mydomain.com (Postfix) with ESMTPS id D1A8212021B
for ; Fri, 29 Apr 2016 14:51:00 +0300 (MSK)
Received: by mail-lf0-f50.google.com with SMTP id j11so131086508lfb.1
for ; Fri, 29 Apr 2016 04:51:00 -0700 (PDT)
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
d=1e100.net; s=20130820;
h=x-gm-message-state:mime-version:reply-to:date:message-id:subject
:from:to;
bh=p+mmTW1hPWVgJY5gdnhI1FVxrF+tV0EdNaJGsNSB/rA=;
b=GhdkhxTo3gHHP2RgAE+csoeuoBBatdYEThJGPRHnnXNzDGZQqiduheHSMD4ZNa5Nyx
IzisvQLKLOZVr7wj/4HioTIq25wC4St1tMFK2XrXEWHvPk3Fb4CqAUOIE82/cJuV7G1B
fW1yT2E9rp2hKj0jh14+62yN3/sLS3RRLTi1G0LS2zJUCh9fGohHptfDvAhvnuixMB5A
4eKDj66Cg48u9YpW3iV8CXjVtKLX4nIL6syca7+w1xqWMrsF2rtVRLjc9JMmGz5/CV27
pHr4qZ9cQ5lsPlNgL694nupVWzMHtZ3GYCFL+8/lu/2gGvrC07ckYe7zbSU7Q9OoQCsk
mvDA==
X-Gm-Message-State: AOPr4FXmOxKf+u2eSjw8EEcPEv1n0vn4jvZCfHTtEO2OnfY1bD5esCGscn+VaWBolaQc9w==
X-Received: by 10.112.72.42 with SMTP id a10mr1093893lbv.111.1461930659912;
Fri, 29 Apr 2016 04:50:59 -0700 (PDT)
Received: from mail-lf0-f52.google.com (mail-lf0-f52.google.com. [209.85.215.52])
by smtp.gmail.com with ESMTPSA id z8sm2227525lbv.42.2016.04.29.04.50.59
for
(version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
Fri, 29 Apr 2016 04:50:59 -0700 (PDT)
Received: by mail-lf0-f52.google.com with SMTP id j11so131085989lfb.1
for ; Fri, 29 Apr 2016 04:50:59 -0700 (PDT)
MIME-Version: 1.0
X-Received: by 10.112.235.195 with SMTP id uo3mr7269922lbc.18.1461930659172;
Fri, 29 Apr 2016 04:50:59 -0700 (PDT)
Reply-To: [email protected]
Received: by 10.25.83.10 with HTTP; Fri, 29 Apr 2016 04:50:59 -0700 (PDT)
Date: Fri, 29 Apr 2016 14:50:59 +0300
X-Gmail-Original-Message-ID:
Message-ID:
Subject: GUGOGOGIOG
From: "[email protected]"
To: [email protected]

Answer 1

and what domain is registered in the postfix settings?
you can also see what they say here https://www.mail-tester.com/
they describe the problems in sufficient detail
and what is the PTR of your IP?

Answer 2

Please show the full title of the email you received.

Answer 3

Your letter is strange, I wonder how you formed it.
It is not SPF that is failing for you, it is DMARC for the gmail.com domain that is failing for you. DMARC is reporting because the email is not signed with DKIM, although it was sent via GMail's web interface. And DKIM it is not signed, most likely because From: is formed with an error.
there should not be double quotes. Or
or
PS And SPF mydomain.com does not affect this letter at all, i.e. SPF is checked for the gmail.com domain and your IP is not there, of course, it will always fail on redirects if you do not rewrite the envelope address.