Who is using SPF?
Many major ISP's (Internet Service Providers) are now publishing SPF records as well as enforcing them. Here is a small list of the many that are using SPF:
As you can see, these are some major players in internet services.
Why is SPF needed?
sheer amount of spam being transmitted daily is growing exponentially
and one of the most widely used methods of sending spam is by forging
or "spoofing" the sender's email address (making a mail server think
that an email is sent from one person/domain when it is actually sent
by a different person/domain). SPF allows your mail servers to easily
distinguish forged and spoofed email from real mail. More importantly,
SPF works before the message body is transmitted, saving you
the bandwidth cost of downloading the message and the CPU cost of
filtering it. Some large ISP's are already requiring SPF records to be
published before they will white list the Web Host. This policy helps
keep their end users from getting spam and also helps Web Hosts because
the ISP can block an individual domain versus a Web Host's entire mail