I work for the alumni office of a Dutch university and every quarter we send out an alumni newsletter (with Tripolis).

Our "subscriber" list is an opt-out list, and we send out around 50,000 emails per edition. Our average open rate is 22.97%.

However, the open rate for hotmail.com is 31.47% while for gmail.com it's 22.90%. These are our top two domains, respectively 23.27% and 19.78% of our subscribers. I yet have to understand why there is such a huge difference between these two domains.

I did notice that gmail users hardly open the newsletter in a browser on a personal computer, but instead they use an email client whereas hotmail users do not. Smartphone opens are high on both.
Both hotmail and gmail show images by default, so that's not the reason.

Demographic differences: hotmail seems to be favoured by Dutch (speaking) alumni. But our newsletter is in Dutch and English, and the English version even has a slightly higher open rate than the Dutch one.

Gmail shows our newsletter in the Promotions tab, and it shows "Message clipped" instead of the footer (which has the unsubscribe link).

Can the Promotions tab account for a 8.57% difference? http://kb.mailchimp.com/delivery/deliverability-research/about-gmail-tabs claims it should not, though "higher education" was not represented as an industry in the study they refer to.

Another assumption is that gmail is more often used as a "spam" account. I myself also have a gmail account that I never open.

Any ideas?