Marketing lists and remote content
Don’t rely on ‘percentage opens’ as an entirely accurate metric.
To elaborate: the way most mailing lists determine whether a message was opened is by embedding a small, usually transparent, image in the HTML which is loaded from a remote web server when the message is viewed. That then indicates to the list admin the message has been ‘opened’.
However, it relies on certain assumptions being true:
1) Your reader has enabled HTML mail rather than just plaintext.
2) Their email client has permissions to download remote content when viewing an email.
In a corporate environment, it’s entirely possible that at least one of those might not be true – we certainly advise our clients not to allow automatic downloading of remote content in email, as it can be an easy attack vector for the usual suspects of internet unpleasantness (malware, virii, etc.).
Even outwith the corporate environment, people who are concerned about their privacy might well disable one or both of those ‘features’ – I know I always make sure to read my email in plaintext, for example.