If you have a domain and don't want to be blocked for sending spam.
If you're a proud owner of your own domain (say,
and have an email address with it,
then you probably know what are SPF-records and how to use them
to prevent other people getting unwanted messages with your address
in the "From" field.
But one day you may learn (usually by getting a "bounce" email) that
spammers can send emails from subdomains of your main domain
That's not nice, because: a) other people are getting spam, and b) your domain gets visible by anti-spam filters.
To somehow prevent or limit this, you can simply add this wildcard SPF record to your DNS system:
* IN TXT "v=spf1 -all"
It will tell all email servers that respect SPF that they should reject all mail coming from all subdomains of your domain.