Little Snitch, and your mail program, and your wife, are all looking out for you.
Spammers send out millions of emails, most of which are never seen because they go to accounts that never existed, no longer exist, or exist but are no longer actively read. Naturally, they get the highest return on their investment by concentrating on email accounts that someone actually reads. But how to tell...
Well, if someone responds to the email, then presumable they read it. That's wonderful, but doesn't happen often. There's still value in all the accounts where human eyeballs actually see the mail, even if they don't respond. They'd love to be notified when the message gets opened.
And there is a way. All they have to do is embed a link to an image in the email. Your mail program won't actually download the image until you view the email. The act of fetching that image is the notification they're looking for.
Each piece of email they send out is customized; the email address (or equivalent identifying data) that was used has been appended to the url for each image (actually, for each link, just to be sure). The act of viewing the image (or clicking on anything clickable) informs them immediately that you're eyeballing their missive, and your email address suddenly turns golden in their list.
You've done it now. The spammers now know that you're someone who actually reads their spam. Your email address has just gone up in value, and has probably already been sold on that basis.
Your wife tried to shield you, Little Snitch tried to shield you, your mail program tried to shield you, but you didn't listen. Let the spam roll in.
