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SPF-record / E-mail marked as spam in Gmail [solved]

E-mail send from my own e-mail (example@domain.nl) to a Gmail-account is marked as spam. When I view the e-mail headers you will see a softfail. I have tried to set the SPF-record for my domain, but I keep getting a softfail:

Received-SPF: softfail (google.com: domain of transitioning example@domain.nl does not designate 2a02:348:83:cc40::1 as permitted sender) client-ip=2a02:348:83:cc40::1;

2a02:348:83:cc40::1 =

I have tried to set different SPF-records for the domainname: example.nl, an example:

v=spf1 ip4: ~all

What SPF-record to I need to set in order to 'pass' for the SPF?