I need to verify that my email actually is read. And not stuck in spam or trash. Any system for verification out there?

My clients register online. Then i send them all information they need by email. Im using salesforce and mail is sent from there by google apps. This works pretty good, but every now and then i loose customer becouse they did not receive any info. Most of the time its becouse it is in the spam folder. My question, how can i know they got the info?! And, i also would like to know if they UNDERSTOOD the info... Do YOU got any good system for this? Some kind of verifiation? Some link they must click on, some action they must do... i need a foolproof system, so i can be 100% sure that THIS people read the info, i know they pay me - this people did not.. "hey, i better call them now, og i loose money!" Any ideas? Thanks for reading.