I write many proposal email (not SPAM) and most of them as are send to generic emails like " info(at)name_of_the_company.com" or people that just do not monitor their mail or for any other reason, resulting in a lot of effort that goes unanswered.
I end up forgetting and I would like to have a function in Thunderbird that when I request a "Return receipt" and the receiver do not reply I can just bring them up to follow up.