The rate data are noisy, but a linear fit over the last 12 days says the
current rate per day is about 2500 and the acceleration is about 41 perday per
day. Extrapolating this forward using simple numeric integration shows the
number at about 600,000 domains at the end of the year.
This ignores records that have not been recorded at
http://www.spftools.net/register.php . I know that there are at least 120,000
domains with SPF that have not been recorded. The fellow who did the work
emailed me after one of my earlier posts on this subject. He did try to submit
them to the registry, but, for whatever reason, they have not been recorded.
The conclusion is that the number showing at the registry is a very
conservative lower limit.
Mark Holm