Years ago, I used to get marketing spam emails from Bank of America. In their email, they did not offer a way to opt out from those types of email, so I invalidated the unique email address that I had created just for them. A few months later, I got a snail mail letter like the one Dan got, telling me that emails were being rejected and that I needed to correct my email address. I went through the same sort of nonsensical dialog with them, and they simply would not let me opt out from their marketing emails, so I left it disabled for a few years. Eventually they offered "email preferences", so I re-enabled it.

My wife continues to get spam snail mail from Citi, and they offer no way to opt out. If it was my account, I would switch banks.

Back to the main topic: I think it's pretty stupid of the HSBC IT folks to assume that an email was not read because the tracking pixels were never accessed. Lots of email clients these days do not load images by default.