SPF stands for Sender Policy Framework.
It's a way to tell the Internet which servers are allowed to send email from your domain. Many email providers are more to likely deliver email coming from your domain to the recipient inbox if you have a valid SPF record.
To use SPF, you must publish a record associated with your domain name that lists all the providers that are allowed to send email from you. A typical record might look like this:
example.org -> v=spf1 a:spf.mayfirst.org include:_spf.protonmail.ch ~all
This record means that the domain, example.org, only uses May First and Proton Mail to send email. If anyone sends an email claiming to be from any @example.org address but is sent from a different server, then it should be treated with suspicion.
Setting an SPF record is tricky because there is no one size fits all. Most organizations send email from a variety of servers, including their main email provider, any bulk mail provider and even directly from their web site.
If you would like to set an SPF record for your domain name, please follow our guide on preparing your domain name to send email.