Checking if a message is marked spam or not isn't the best way to confirm a legal delivery. Say, I setup my Gmail to forward all mails to my thunderbird or outlook but when it gets there, the respective clients accidentally filter the specific message as spam, what then?
Monitoring for a "read bit" on the message is the only way to confirm proper delivery.
Monitoring for a "read bit" on the message is the only way to confirm proper delivery.