You asked about "open rate" and for the folk I know, all have blocked read response replies. Also, most of us know use adblockers so anyone that sells you this, you are getting ripped off.
My brother does use Constant Contact and MailChimp but I worry you are asking how to spam.
Even so, this thread is now a link free zone to curb spammers.
Hello email marketing experts, I'm starting my new business in email marketing using a custom software and Google G suite. Unfortunately, I didn't find any solution to track my emails such as open rate, click rate, conversion, unsubscribe, spam, bounce... etc.
Could you help me please guys? Thank you in advance!