The domain is used only for mechanisms such as a bare 'a' mechanism that have an implied domain. It can also be used for syntax checking of records with more complex macros (although this has not been thoroughly tested yet).The %d macro is extracted from the supplied mail from address.Tests the supplied SPF record to see if it is valid. Except for %d, does not currently support records that include macros.This test does NOT look up the record for the supplied domain. This test is for checking the syntax of records before you publish them. This test is for evaluating the performance of your record based on different IP addresses that mail might come from (this is the IP address of the mail server).
Simple and safe workaround for this is using strlen() before filter_val().Option #1 is to track the event on your thank you page anyway, and just leave out the form properties.