You could start is offering facts about the process. The vast majority of folk having issues sending and receiving "some" mail are those with custom / personal domains that are simply not configured correctly for the ever more stringent delivery requirement of providers like google. Others are relying on the skills of some of the USA finest internet service providers that have been making a mess of email for as long as I have used email. Yet other folk have fancy email signatures that include images their recipient has identified as Spam related (usually downloaded blue Fs and little stylized Ts Others include tracking information in their email which send the receivers privacy tools spare and they drop the email. Really your question as stated is like the perennial "how long in a piece of string?"