However, a managed e-mail service isn't how most people have e-mail. Most people use HTML e-mail that retrieves the mail in real time, so the only way to access it is to read it.
Regardless, most e-mail receipt triggers that are not service specific these days are done using transparent pixels that are downloaded in a managed mail settings, or loaded when the e-mail is opened in HTML e-mail. They are strictly meant to trigger a read notice since the only reason they are downloaded/loaded is if the e-mail is accessed, they are single purpose, unique pixels that are automatically generated for each e-mail sent. The only way to avoid such "read" receipt triggers these days is to allow only UTF-8 text to load in an e-mail and nothing else. Of course, since most e-mails these days are formatted in HTML, the message will look quite screwed up since the HTML will be read as text and not rendered. But, it is literally the only way to avoid being tracked.