SPF (Sender Policy Framework) is a kind of DNS record, very important and mandatory in order to fight spam and increase the deliverability of your email messages.
It's a TXT record, which specify a list of authorized SMTP servers which are allowed to send messages from your domain.
SMTP servers can be specified with their hostnames or by ipv4 and ipv6 numbers.
This as example one of our default SPF:
"v=spf1 a mx ip4:89.38.98.235 ip6:2a00:7c80:0:1e0:0:0:0:235 ~all"
There are also free tools and resources to help you, here it is some:
SPF check: https://mxtoolbox.com/spf.aspx
SPF Record Syntax: http://www.openspf.org/SPF_Record_Syntax
SPF Wizard: http://www.spfwizard.net/
Common mistakes when creating a SPF record: http://www.openspf.org/FAQ/Common_mistakes
SPF Record testing Tools: http://www.kitterman.com/spf/validate.html
