What SPF does
SPF lets receiving mail servers check whether an outbound server is authorized to send mail for your domain. The domain publishes a DNS TXT record listing permitted senders.
Syntax basics
SPF TXT records start with v=spf1, list mechanisms (include, a, mx, ip4), and end with an all mechanism such as -all or ~all.
One TXT per SPF
You should publish only one SPF TXT record for a given domain or label. Multiple SPF TXT records invalidate SPF for many receivers.
Related reading
After SPF is stable, align reporting with DMARC explained.