Unfortunately, most of the images are downloaded from remote servers as you open the emails. This lets newsletter publishers know you're indeed interested and open their emails. It also tells spammers you received their emails and, approximately, where in the world you are.
If you want to forego these revelations and spare your data plan the additional bytes, you can tell iPhone Mail not to download remote content when opening emails.
Prevent iPhone Mail from Downloading Remote Images
To make sure iPhone Mail does not download remote images in emails (which would reveal your opening the message and your approximate location, too):
- Go to the Home screen.
- Open Settings.
- Tap Mail, Contacts, Calendars.
- Make sure Load Remote Images is not enabled.