I would just block all images loaded from external URLs, since any of them could be used for tracking, but the newsletters are less my concern, it's more the individuals that I would be concerned about, and they tend to be a almost-fully plain text e-mail with a 1x1 image from one of many well-known domains for tracking.
Emails can have "inline" attachments, where an image can be attached in the body of the email itself. So the image shows up without any network access, but it's generally base-64 encoded (costing 4x space) and ends up using a lot of email storage space.
Ohh, I was thinking 256/64 = 4x, but it's apparently it's 8 bits / 6 bits = 1.3333x. Thanks. Plus for email a little extra for a =\r\n on every line, so more like 1.4x.
